OAL and FPS

[6.5x300]

Hey guys need some advise here as I had an interesting discovery shooting today. I found that as I seated the bullet deeper shortening the OAL my fps dropped. This seemed like backwords logic to me as I would have thought the fps would increase from less space in the case for powder to expand. What may explane this outcome is that I started with the bullet seated into the lands and the more the bullet jumped the less the fps.

My equipment was .338 edge with 30 pac-nor savage 110 action fire formed nosler 300 RUM brass 92 H-1000 CCI 250 primer. All grouped less then .6 as I'm still practicing and usuall get that one flier.

OAL 3.965 = 2885 fps ( maybe .05 into the lands I'll have to check)
OAL 3.910 = 2870 fps (not in the lands so the .05 would be about right)
OAL 3.875 = 2870 fps
OAL 3.840 = 2855 fps (best group .3)

Also got the same results from my M94 6.5X55 sweed
Norma brass max load of RL 22 CCI BR primer Hornady 140 SST

OAL 3.210 = 2545 fps
OAL 3.375 = 2605 fps (not quite touching the lands, rifleing, groves, you know what I mean)

Kind of curious as to everyones thoughts on this.

Thanks,

BT Sniper

Shot .3 with the edge, the 6.5 I need to work on

 

[woods]

What you experienced is entirely normal. Pressure and velocity generally decrease the further from the lands you load. Reference this chart by Barnes

Brief explanation;


the blue bars are ball powders,

the red bars are extruded,

the first set of bars above the caliber are at .025" off,

every set of bars goint to the right in that caliber is additional .025" off,

except 300 wtby where they couldn't get close to the lands


The 308 with ball powders shows decreased pressure with increased seating depth

The 308 with extruded powders shows a general decrease in pressure until seated .125" off lands

The 30-06 with ball powders shows a drastic decrease in pressure between .050" and .075" and then a slight increase

The 30-06 with extruded powder show a steady decrease in pressure with increased seating depth

The 300 WSM with ball powder shows a general decrease in pressure with increased seating depth

The 300 WSM with extruded powder shows a decrease until .075" off and then goes back up

The 300 win mag with ball powder shows a steady decrease in pressure with increased seating depth

The 300 win mag with extruded powder shows an increase between .025" to .050" and then a general pressure decrease with increased seating depth

The 300 Wtby shows a steady decrease in pressure with increased seating depth with both ball and extruded powders

The 300 RUM with ball powders shows a steady decrease in pressure with increased seating depth

The 300 RUM with extruded powders shows an up and down but generally steady pressure

The point is that in several of these instances the pressure is reduced with increased seating depth and that seems to be a general trend shown on this chart.


Also, on the Hornady site Hornady at the bottom of the page:

"To illustrate the effects of variations in bullet travel before the bullet enters the rifling, we'll compare a standard load with adjustments made only in the bullet's seating depth.

In a "normal" load with the bullet seated to allow about one 32nd of an inch gap (A) between the bullet and the initial contact with the rifling, pressure builds very smoothly and steadily even as the bullet takes the rifling. Pressure remains safe throughout the powder burning period (B), and the velocity obtained - 3500 fps - is "normal" for this load in this rifle.

Seating the bullet deeper to allow more travel before it takes the rifling, as in these next two illustrations, permits the bullet to get a good running start (C). Powder gases quickly have more room in which to expand without resistance, and their pressure thus never reaches the "normal" level. Nor does the velocity; with the same powder charge it only comes to 3400 fps (D).

When the bullet is seated to touch the rifling, as in the accompanying illustrations, it does not move when the pressure is low (E); and not having a good run at the rifling as did the other bullets, it takes greatly increased pressure to force it into the rifling. As the rapidly expanding gases now find less room than they should have at this time in their burning, the pressure rise under these conditions is both rapid and excessive (F). Velocity is high at 3650 fps - but at the expense of rather dangerous pressure.
Many rifles deliver their best groups when bullets are seated just touching the rifling. Seating bullets thus can be done quite safely if the reloader will reduce his charge by a few grains. The lighter load will still produce the "normal" velocity without excessive pressure.

This brings up another pointer on accuracy for shooters who may have a few thousand rounds through their rifle barrel and have noted a fall off in the accuracy they can obtain with their standard loads. Hot gases from the shots previously fired through the barrel erode the throat and thus increase the distance a bullet must travel before contacting the rifling. By loading longer bullets and seating them farther out so they'll touch the rifling - making powder charge adjustments as necessary, of course - accuracy can often be improved."

You may have to go to the site to view the pics.

I have experienced the same reduction in velocity when loading for a rifle that can not be loaded close to the lands due to magazine restrictions. The velocity is always lower. One time I seated one bullet with the same exact components and powder charge to get close to the lands instead of to fit the magazine and the velocity increased ~100 fps.

 

[nheninge]

Long story short...

If your car is parked up against a curb and you try to jump the curb from a standing start, you need a lot more gas and effort

If your car is parked 2 feet away from the curb and get a running start it is easier to get over the curb!

Try it... then get your alignment checked

 

[txlongrange]

Long story short...

If your car is parked up against a curb and you try to jump the curb from a standing start, you need a lot more gas and effort

If your car is parked 2 feet away from the curb and get a running start it is easier to get over the curb!

Try it... then get your alignment checked

LOL....nice analogy...straight and to the point.

TXLR

 

[woods]

Whaaatt! Y'all didn't like my long explanation??

 

[6.5x300]

Thanks guys,

It does make sence being closer to the lands generates more pressure. How about seating the bullet deeper to the point of compressing the powder?

For instance in my 300 win mag with max IMR 4350 and nosler 125 bt seated long at 3.50 I get 3550 fps. When I seat it to specs at 3.34 the powder is compressed. I have not shot this over the crony yet but what is everyones thoughts? Will it be increase or decrease in fps.

I have a HS precision barrel that has either a long throat or erosion as I have went .5 over 3.34 and still not hit the lands. It shoots great though.

So this has me thinking what is the advantages to have a long throat in the barrel? More powder capasity correct? To make use of a long barrel and slower burning powder? With the .5" I have to play with in seating depth in both my rifles this could be an additional 2-3 grains of powder capasity yet I'm confused with the pressure variables with case capacity and % of load vs. distance from the lands.

I get that the pressure drops the further from the lands so what happens to the pressure when the % of powder to case capicity is changed by the seating depth of the bullet?

I'll do some more experimenting with the crony. It seems that my belief system has been turned around by these results.

Thanks for your help everyone,

BT Sniper

 

[Mikecr]

Great info WOODS!!

 

[nheninge]

Compressed loads generally generate more velocity because there is more powder. Compression of the load while jamming 68 grains in a 65 grain case (random numbers) while seating your bullet probably generates some extra pressure and thus velocity. Compressed loads are always at the max, and may not be as accurate though!

Simple analogy number 2: Holding a firecraker in your open left hand and place a second firecracker in tight fist. Light them and see which one hurts more! (pirated from Armageddon)(can't claim this one)

Need some other opinions though

OK Woods... Your turn! (Awesome info earlier though!)

 

[txlongrange]

Whaaatt! Y'all didn't like my long explanation??

Woods, your info was good indeed....I just liked nheninge's analogy.


TXLR

 

[woods]

IMO compressed loads do not generate more pressure or velocity because of the powder compression alone. IOW if you had a 30-06 with a load of 63 gr RL22 (compressed) and the bullet was seated deeper in one case and out further in the other (both still with some degree of compression), then the bullet seated closer to the lands would have the higher pressure and velocity. Seems to me that the brass case has very little resistance to ****** pressure and will not create much pressure (like your closed hand example) and the most important things are the size of the combustion chamber and the amount of a running start the bullet has.

So if you had a load that was severely compressed but the bullet was seated .250" off the lands then you would have much less pressure and velocity than a bullet with the same load in the same chamber that was seated .005" off the lands.

That was one of the problems that I had in the 300 win mag with the short magazine that I had to seat .250" off. I had problems with severe compression problems with H1000 and the 200 gr TSX and it would start backing out before I could get a crimp on it. I could get about 78 gr of powder in it and it still only got 2743 fps.

So, firecracker example not as good as car against curb example (for me).

 

[nheninge]

I agree that the brass itself does not generate the pressure because it is too pliable, but in a rifle, it does expand until it comes into contact with the steel chamber wall that has no give and thus pressure is generated indirectly.

Second analogy is like a bad movie sequel, but you still watch it!

 

[6.5x300]

OK lights are starting to come on here for me. Some day I will build a long range 300 win mag. Of course looking at heavy pills (190,208,210 etc.) at max FPS. Lets say I seat the bullet at or in the lands, the variable becomes the OAL in reguards to the length of throat correct? A long throat on this caliber will allow room for more powder, am I on track so far? So a long barrel of 28-30" sould be able to take advantage of the extra powder and generate more FPS as compared to seating the bullet at the standard 3.34" in a standard chamber yet still in the lands?

Simple ? here, will seating the bullet as long as possible allow for greater FPS and still acceptable pressure by increasing the potential powder charge? There for use a heavier load of slower powder like H1000 insted of H4831SC.

Not sure it is simple but thanks for all your help in trying to shed some light in this simple mind.

 

[woods]

You are sorta on the right track but there are some caveats to that. A tighter chamber with a short throat will also generate a lot of pressure with a smaller charge. For instance I have a 338RUM which I had built off a 338 win mag action. The mag was plenty long but not as long as it could have been to allow me to seat a bullet out very far. So I had the gunsmith throat the chamber to allow me to still seat close to the lands and fit the mag which seats the bullet well into the case

Bear with me on this.

This means the size of the combustion chamber (total volume of the chamber) is smaller than a normally sized 338RUM chamber which is known to have a long throat. In all of the RUM's that I have heard about you can not load close to the lands. In mine since I have a smaller combustion chamber I do not have to load as much powder to achieve the same velocity and can not load up to book max. For instance the book max load for RL25 with 225 gr bullets is 98 grains. I get small pressure signs like slight extractor groove marks at 96 gr RL25 and I can load 94 gr RL25 and get the velocity shown for the max load in the book for 98 gr RL25.

So I have a tight chamber including a short throat and can get velocity.

But you are right in that if you have a long throat you can load more slow powder than I could because you would have a larger combustion chamber. It is what Weatherby has used for years to put hot loads in their Wtby calibers. If you were to get a custom chamber in a Wtby caliber then you would not want to get a short throat and shoot factory ammo!!!! NO NO. Could produce enough pressure to blow the gun up.

I will not ever shoot factory ammo in this 338RUM even though it might be possible if the factory loads were a little anemic. Wtby factory loads are loaded HOT to get all that velocity they tout.

What was the question?

Oh yeah

"will seating the bullet as long as possible allow for greater FPS and still acceptable pressure by increasing the potential powder charge? There for use a heavier load of slower powder like H1000 insted of H4831SC."

put it this way: you will NEED to use more powder to generate the SAME fps as you would have been able to achieve with a shorter throat and less powder.

Also

"Some day I will build a long range 300 win mag. Of course looking at heavy pills (190,208,210 etc.) at max FPS. Lets say I seat the bullet at or in the lands, the variable becomes the OAL in reguards to the length of throat correct? A long throat on this caliber will allow room for more powder, am I on track so far? So a long barrel of 28-30" sould be able to take advantage of the extra powder and generate more FPS as compared to seating the bullet at the standard 3.34" in a standard chamber yet still in the lands?"

IMO, the best way to do it is to have the shortest throat that will also allow anyone to shoot factory ammo. The 300 win mag has plenty of case capacity to get enough powder in the case and you can gain velocity easier by loading close to the lands and having a small combustion chamber.

An exception to this would be if you were getting a custom chamber in a caliber that did not have enough case capacity or the case capacity would be reduced by seating a bullet down in the case. For instance I have a custom 6.5 rem mag that I rebarreled off a 300 win mag (long throat short mag Browning BBR) and I had the gunsmith set the lands so I could load those long 6.5 bullets way out there in order to stay out of the powder capacity

Perhaps nhenhinge can come up with a short catchy analogy!! Just kidding.

 

[nheninge]

trilogies are always a disaster!

great post