Throat length, does it really matter that much?

[dkhunt14]

I would say you got the increased velocity simply because you removed the pinch point between the neck of the case and the bullet and had very little to do with the longer OAL and the additional two grains of powder used.

In fact, I would bet that there is a minute donut at the base of the inside of your case necks that was causing more of a problem then the pressure ring on the berger bullets which is very minimal at best.

This is one reason I do not design my reamers to have extremely tight necks. Can cause these little issues that you saw and the more extreme you get in performance the more you will see the effects of these things in velocity and pressure. In the 300 WSM which is about as mild and user friendly as it gets, you do not see much of a problem. In something like a 7mm Allen Magnum or 338 Allen Magnum, it can be dramatic and cause severe pressure swings.

Again, I would bet that if you inside reamed your case necks or had a neck that was 1/2 to 3/4 thou larger in diameter, you would not see this problem with pressuring up. I believe you simply removed the pinch point and that resulted in freeing up some extra performance, very little to do with the actual longer OAL and two additional grains of powder. In my opinion anyway.

These were only fireformed cases and pre turned with a bite into shoulder. There were freshly annealed and turned to .0135 wall for a 338 Neck. They did not have a donut and we only size 3/4 of the neck with a bushing die so that part of the case would not have touched the bullet. Now this is a 17 lb. light gun for 1000 yard BR and we did the same with 2 others. WE throated them from a .210 freebore to about .290. These are all tight necked guns with brass turned to one ten thousandth of an inch so it is not a thick thin brass thing.We also did the same thing with a dasher and it left it left us get to 3025 With 105 VLD's. The pressure went away. The dasher was a 6BR case turned to .010 with a .266 neck. The neck was turned to about 1/16 inch from the case neck junction. Now when fireformed the .010 neck goes back to the middle of the shoulder. No donut. The loads went from 32.5 with pressure to 34 grains and no pressure. A 210 Berger in a WSM with H4350 which is the powder of choice for 1000 yard benchrest usually gives about 2850 tops with a fast barrel. The case is touchier then you think. It goes from good to bolt click in .2 grains of powder. I have shot these two cases for over 6 years now and they are the 2 to shoot for accuracy. They hold about every record in 1000 yard Benchrest. They also win about every aggregate at every 1000 yard range in the country. In Heavy gun I shot 10 shots in 2.815 inches at 1000 and it is the current world record in heavy gun. Matt

 

[Mikecr]

not sure you understand the point of this test, at least your reply does not seem like you did.

I guess I'm suggesting that no test was needed to know the outcome here.
You just tested the same barrels(close enough) and the same bullets at the same pressure, and then declared the resulting same MV -as though a revelation..

107.0 gr, 104% load density(and XXKpsi)(less powder, higher load density).

110.0 gr, 98% load density(and XXKpsi)(more powder, lower load density)
(same pressure)

Same pressure, with all else the same = same MV
QuickLoad would show this.
Why would you think this would turn out another way?
Sounds like madness to me!

 

[Lefty7mmstw]

The only way you will actually get more mv out of a longer throat in a given setup is to allow more run to the rifling. Anything else, as Mikecr has said, is no real surprise at all to me.
I always chuckle when people try to load a 220-230gr pill to the rifling in a 30roy (toss any other heavy pill and long throated round in if you wish) and wonder why they had pressure issues before they topped out with book data. Let her have a run at the rifling and see the difference. Set both barrels' ammo to the same oal and see what happens. Even then, it shouldn't be more than a couple of % better mv at the expense of a lot of powder.

 

[Timber338]

Thanks for sharing your results and taking the time to write this up for us. Although some of you guys think this is obvious, I have always thought that more gunpowder at the same pressure would give more velocity.

Also thanks for your writeup on how you were able to get a rifle sighted in and a drop chart generated with so few shots. I've certainly never been able to do that... I'll see how many shots it takes me with my next new rifle. Guessing it will be much higher than 13.

 

[Fiftydriver]

Thanks for sharing your results and taking the time to write this up for us. Although some of you guys think this is obvious, I have always thought that more gunpowder at the same pressure would give more velocity.

Also thanks for your writeup on how you were able to get a rifle sighted in and a drop chart generated with so few shots. I've certainly never been able to do that... I'll see how many shots it takes me with my next new rifle. Guessing it will be much higher than 13.

I used to do the drop development where you dial in the scope at 100 yards and then go out from there. Only problem is that a minute error in 100 yard zero will translate to a significant error as range increases.

So, I decided to reverse engineer the process. If you have good shooting conditions, I personally feel that if you zero at longer ranges, you can remove most if not all the error at long range and that makes it much quicker to set up a rifle.

Now, that is all dependent on a good accuracy BC, good accuracy velocity,and all other data to develop the trajectory.

This works best for hold over shooting but as shown in this case it works very well for dial up shooting as well. Remember this is just to get a rifle CLOSE. All rifles including this one will need more range time to tweak the drop chart to match up perfectly but it shows that getting things close does not take many rounds at all.

That does not mean this rifle is ready to go to the field and hunt with.

 

[Fiftydriver]

I guess I'm suggesting that no test was needed to know the outcome here.
You just tested the same barrels(close enough) and the same bullets at the same pressure, and then declared the resulting same MV -as though a revelation..

107.0 gr, 104% load density(and XXKpsi)(less powder, higher load density).

110.0 gr, 98% load density(and XXKpsi)(more powder, lower load density)
(same pressure)

Same pressure, with all else the same = same MV
QuickLoad would show this.
Why would you think this would turn out another way?
Sounds like madness to me!

How many times have we read here on LRH where someone had a given chambering but with a long throat and they were reporting velocity levels far over what was normal for that chambering and claiming that it was because of the longer throat allowing the use of much more powder to be used?

When in fact it was simply because of the amount of powder they were using and pushing pressured over where they should be.

The point of this test was simply to show that the length of the throat has very little to do with peak velocity in the whole scheme of things. Never made claims that it was a revelation of any kind, only offered the information so that someone could see the actual results of comparing two different throat lengths and that if your getting much more velocity with a given throat length, its likely because your simply running high pressures.

To MANY here on LRH, especially recently, this would be a revelation. While I was not expecting a lot of difference in velocity, throwing opinion out there is much different then posting actual test results that proves the opinion. That is all I did.

 

[Fiftydriver]

These were only fireformed cases and pre turned with a bite into shoulder. There were freshly annealed and turned to .0135 wall for a 338 Neck. They did not have a donut and we only size 3/4 of the neck with a bushing die so that part of the case would not have touched the bullet. Now this is a 17 lb. light gun for 1000 yard BR and we did the same with 2 others. WE throated them from a .210 freebore to about .290. These are all tight necked guns with brass turned to one ten thousandth of an inch so it is not a thick thin brass thing.We also did the same thing with a dasher and it left it left us get to 3025 With 105 VLD's. The pressure went away. The dasher was a 6BR case turned to .010 with a .266 neck. The neck was turned to about 1/16 inch from the case neck junction. Now when fireformed the .010 neck goes back to the middle of the shoulder. No donut. The loads went from 32.5 with pressure to 34 grains and no pressure. A 210 Berger in a WSM with H4350 which is the powder of choice for 1000 yard benchrest usually gives about 2850 tops with a fast barrel. The case is touchier then you think. It goes from good to bolt click in .2 grains of powder. I have shot these two cases for over 6 years now and they are the 2 to shoot for accuracy. They hold about every record in 1000 yard Benchrest. They also win about every aggregate at every 1000 yard range in the country. In Heavy gun I shot 10 shots in 2.815 inches at 1000 and it is the current world record in heavy gun. Matt

I have never found the 300 WSM case to finicky in any way but again, I do not build tight throated rifles. My theory is that most accuracy is developed in the throat area of a chamber so as long as the neck to chamber fit is not SLOPPY and the chamber is on axial alignment to the bore, accuracy will be great and I have not been proved wrong yet. Again, I build long range big game rifles, not BR rifles.

So your the current world record holder for 1000 yard group? Congrats and fine shooting!

 

[Fiftydriver]

The only way you will actually get more mv out of a longer throat in a given setup is to allow more run to the rifling. Anything else, as Mikecr has said, is no real surprise at all to me.
I always chuckle when people try to load a 220-230gr pill to the rifling in a 30roy (toss any other heavy pill and long throated round in if you wish) and wonder why they had pressure issues before they topped out with book data. Let her have a run at the rifling and see the difference. Set both barrels' ammo to the same oal and see what happens. Even then, it shouldn't be more than a couple of % better mv at the expense of a lot of powder.

Freebore is a great thing to flatten a pressure curve as you mention. That's how Roy Weatherby made his money. Sadly, it can have very poor effects on accuracy depending on how the throat is set up as far as diameter.

This system works great for most chambering and most bullets but you do reach a certain point in velocity and in bullet when jumping a gap to the lands will result in very poor results.

Most of my wildcats are prime examples of this. Running a really long, heavy bullet into a fast twist barrel will wring it out like a wet towel and as a result the jacket/core bond will weaken. What happens is that on these heavy, very long bullets, the front half of the bullet will engage the lands and begin to rotate but because the bullet is so long and heavy, the rear of the bullet will resist this rotational force and it takes it toll on the jacket/core bond and then bad things can happen as far as accuracy goes. Again, with MOST chamberings that produce less then 3200 fps with these heavy long bullets, generally you do not see problems. But when you take the same bullet and slam it into the lands and then rev it up to 3400-3500-3600 fps and at best, you have very poor accuracy results, at worse, you have total bullet failure when the bullet leaves the muzzle because of the stresses from slamming the bullet into the lands and then running the velocity up and things come apart.

The only cure for this is a heavier jacket to handle these high stress loads.

In full custom rifles, that have a throat diameter that matches the bullet diameter very closely, I have not seen much accuracy drop off with freebore until you reach the point that the stresses imposed on the bullets cause the jacket/core separation mentioned previously.

One thing I do not like about long freebore is that you can see more flame erosion in the throat as there is more room for the hot gases to slip past the bullet in the throat. This will erode a throat faster and the longer the throat, the more distance a bullet has to travel before it engaged the rifling. As such, accuracy can drop off faster with a long throat then with a short throat.

Most match bullets recommend seating to or just barely off the lands for best accuracy. In fact Berger bullets usually shoot the best seated into the lands. For a hunting rifle I do not recommend that but that is what they recommend for best accuracy potential no matter if its an 80 gr 224 cal bullet or a 300 gr 338 cal bullet.

 

[Ridgerunner665]

A bit of a coincidence that you brought this up...

Just last night I noticed the same thing while doing some experimenting on QuickLoad (no shots fired)...equal pressure gave just a few fps more speed regardless of seating depth being longer...and as you (Kirby) mentioned, the only benefit to loading long is if you can gain enough capacity to move to a different powder....I'm also one who prefers to keep load densities below 107%.

60,200 psi was the target pressure...

Cartridge          : .280 Ack Imp Nosler brass
Bullet             : .284, 160, Nosler AccuBond 54932
Useable Case Capaci: 64.820 grain H2O = 4.209 cm³
Cartridge O.A.L. L6: 3.340 inch = 84.84 mm
Barrel Length      : 24.0 inch = 609.6 mm
Powder             : Norma MRP

Predicted data by increasing and decreasing the given charge,
incremented in steps of 0.792% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
 %       %    Grains   fps   ft.lbs    psi    psi      %        ms

-07.9   93    58.11   2777    2741   46763  12051     98.5    1.330
-07.1   94    58.61   2802    2790   48081  12136     98.7    1.313
-06.3   95    59.11   2827    2840   49440  12218     98.9    1.296
-05.5   96    59.61   2852    2891   50843  12296     99.1    1.279
-04.8   96    60.11   2877    2941   52287  12370     99.3    1.263
-04.0   97    60.61   2902    2992   53778  12440     99.5    1.247
-03.2   98    61.11   2927    3044   55313  12505     99.6    1.231  ! Near Maximum !
-02.4   99    61.61   2952    3095   56900  12566     99.7    1.216  ! Near Maximum !
-01.6  100    62.11   2976    3147   58530  12623     99.8    1.200  ! Near Maximum !
-00.8  100    62.61   3001    3200   60204  12675     99.9    1.185  ! Near Maximum !
+00.0  101    63.11   3026    3252   61924  12723    100.0    1.170  ! Near Maximum !
+00.8  102    63.61   3050    3305   63695  12766    100.0    1.156  ! Near Maximum !
+01.6  103    64.11   3074    3358   65520  12805    100.0    1.141  !DANGEROUS LOAD-DO NOT USE!
+02.4  104    64.61   3099    3412   67399  12840    100.0    1.127  !DANGEROUS LOAD-DO NOT USE!
+03.2  104    65.11   3123    3465   69336  12875    100.0    1.113  !DANGEROUS LOAD-DO NOT USE!
+04.0  105    65.61   3147    3519   71333  12908    100.0    1.099  !DANGEROUS LOAD-DO NOT USE!

Results caused by ± 5% powder lot-to-lot burning rate variation using nominal charge
Data for burning rate increased by 5% relative to nominal value:
+Ba    101    63.11   3099    3411   68992  12437    100.0    1.120  !DANGEROUS LOAD-DO NOT USE!
Data for burning rate decreased by 5% relative to nominal value:
-Ba    101    63.11   2936    3062   55035  12860     98.8    1.228

Cartridge          : .280 Ack Imp Nosler brass
Bullet             : .284, 160, Nosler AccuBond 54932
Useable Case Capaci: 66.418 grain H2O = 4.312 cm³
Cartridge O.A.L. L6: 3.440 inch = 87.38 mm
Barrel Length      : 24.0 inch = 609.6 mm
Powder             : Norma MRP

Predicted data by increasing and decreasing the given charge,
incremented in steps of 0.787% of nominal charge.
CAUTION: Figures exceed maximum and minimum recommended loads !

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
 %       %    Grains   fps   ft.lbs    psi    psi      %        ms

-07.9   92    58.57   2763    2713   45775  12210     98.4    1.345
-07.1   92    59.07   2788    2761   47041  12298     98.6    1.327
-06.3   93    59.57   2812    2810   48343  12383     98.8    1.311
-05.5   94    60.07   2837    2859   49682  12464     99.0    1.294
-04.7   95    60.57   2861    2909   51061  12541     99.2    1.278
-03.9   96    61.07   2886    2958   52484  12614     99.4    1.262
-03.1   96    61.57   2910    3009   53950  12682     99.5    1.247
-02.4   97    62.07   2934    3059   55453  12747     99.7    1.231  ! Near Maximum !
-01.6   98    62.57   2959    3110   56995  12807     99.8    1.216  ! Near Maximum !
-00.8   99    63.07   2983    3161   58578  12863     99.9    1.201  ! Near Maximum !
+00.0  100    63.57   3007    3212   60205  12915     99.9    1.186  ! Near Maximum !
+00.8  100    64.07   3031    3264   61878  12962    100.0    1.171  ! Near Maximum !
+01.6  101    64.57   3055    3316   63600  13005    100.0    1.157  ! Near Maximum !
+02.4  102    65.07   3079    3368   65372  13044    100.0    1.143  !DANGEROUS LOAD-DO NOT USE!
+03.1  103    65.57   3103    3420   67196  13080    100.0    1.129  !DANGEROUS LOAD-DO NOT USE!
+03.9  103    66.07   3126    3473   69074  13117    100.0    1.115  !DANGEROUS LOAD-DO NOT USE!

Results caused by ± 5% powder lot-to-lot burning rate variation using nominal charge
Data for burning rate increased by 5% relative to nominal value:
+Ba    100    63.57   3081    3372   67014  12634    100.0    1.135  !DANGEROUS LOAD-DO NOT USE!
Data for burning rate decreased by 5% relative to nominal value:
-Ba    100    63.57   2917    3023   53532  13034     98.7    1.245

 

[Lefty7mmstw]

Kirby, I believe I am running into the high jacket stress issues you have mentioned with the 171 barnes in my 7stw and 7rum. They both appear to love the bullet in cool/cold weather, but shoot very poorly at warmer temps with this bullet. In contrast, my 7rem shot the 171 barnes very well with a mild load no matter the temp. I am not seeing this issue with either rifle with hdy interlocks, berger target, or nos bonded.

personally, I'll let my rifles have their freebore and load to mag. restrictions, as if you build the load to be accurate with the freebore the sky cannot suddenly fall in on you... If she shoots, she shoots.

As to the small volume increase related to longer oal, remember that you only get roughly 25% of the case capacity increase in increased speed. So a 1-2% increase in case capacity is translated into jack squat... it's lost in the noise.

 

[Jay Kyle]

I model this in Quickload frequently and what Kirby sees is pretty common in the models. You need to work hard to take advantage of the extra capacity. Don't forget to reduce the start pressure if your freebore is similar to Weatherby specs and you jump to the lands.

Jay

 

[dkhunt14]

The only way you will actually get more mv out of a longer throat in a given setup is to allow more run to the rifling. Anything else, as Mikecr has said, is no real surprise at all to me.
I always chuckle when people try to load a 220-230gr pill to the rifling in a 30roy (toss any other heavy pill and long throated round in if you wish) and wonder why they had pressure issues before they topped out with book data. Let her have a run at the rifling and see the difference. Set both barrels' ammo to the same oal and see what happens. Even then, it shouldn't be more than a couple of % better mv at the expense of a lot of powder.

This is not what I have seen over the Chronograph. I shoot all my VLD's 10 to 15 in no matter if it is a 6 Dasher or a 300 WSM. I got increases in velocity and less pressure with throating the guns out and keeping the bullet in. This was done with at least 3 Dashers and a bunch of WSM and most of the time it was close to 100 feet gain. How can a program predict when it doesn't know how tight the barrel is? I have seen a hundred or more feet difference in the same brand. Seen more by going to a different brand. How does it know if my Bergers are .3079 or .3086 Diam. That changes velocity and pressure also. I believe they can be helpful but are not absolute. My chronograph and drops for 1000 yards verify this. Matt

 

[zfastmalibu]

My thoughts, Y
You can't compare two different barrels regardless of bore size at the breech and muzzle. There are too many variables.
Increasing the throat length gives more case volume, the same as improving a case does. There really is no debate here, its fact. Now the amount of extra velocity you will see in a 100 grain case vs a 30 grain case will be drastically different.
Alex

 

[Jay Kyle]

This is not what I have seen over the Chronograph. I shoot all my VLD's 10 to 15 in no matter if it is a 6 Dasher or a 300 WSM. I got increases in velocity and less pressure with throating the guns out and keeping the bullet in. This was done with at least 3 Dashers and a bunch of WSM and most of the time it was close to 100 feet gain. How can a program predict when it doesn't know how tight the barrel is? I have seen a hundred or more feet difference in the same brand. Seen more by going to a different brand. How does it know if my Bergers are .3079 or .3086 Diam. That changes velocity and pressure also. I believe they can be helpful but are not absolute. My chronograph and drops for 1000 yards verify this. Matt

..for Quickload predictions, I believe you've answered your own question = "I believe they can be helpful but are not absolute". Modeling is and always was an approximation that helps cut back the noise in a study so you can focus your empirical efforts on meaningful testing.