Chamber Leade, Seating Depth, Powder Capacity, Pressure

[Chester Evans]

In the May 2005 issue of Rifle, Nosler tested a standard SAAMI spec 300 WSM and a second barrel chambered with the same reamer except for a straight 0.250 inch leade.

The thought was to determine if seating bullets out to 2.880", 2.90" and 3.0" and the corresponding additional powder capacity would allow additional velocity gains without overpressure. They used an Oheler Model 83 (I thought they only made a 43?) for all pressure and ballistic measurments.

They matched SAMMI pressures (factory loads) using H-48310sc(69 gr), and RL-22(70.5 gr) at the SAAMI OAL of 2.82 inches (59,000-60,000 psi). They then increased the bullet OAL to 2.880", 2.90" and 3.0" using the same loads as above. What they found was that even though the load remained the same, the pressure increased above SAAMI spec levels as the bullet was seated to 2.880", 2.90" and 3.0". This was true even though the it (bullet) was not touching the lands yet (maybe it touched at 3.0"). They also mentioned that they did not believe that using a slower powder would help

I am just getting into the reloading game and I don't know enough of the variables to know if what they are saying is true. I am sure the top gunsmiths like Chris, Kirby, etc. must have some knowledge of this since they are deciding how much leade to provide, OAL, setoff from the lands, etc.

I am sure this topic must be of interest since it seems most on this site are building toys that press the long-range edge. The reason I am asking is I want to build a 6.5mm using a short action 2.92" max OAL length and I would like to see if I can get 3000-3100 fps from a 24" barrel with a 142gr SMK. Am I dreaming or would it be pushing the pressure envelope? I don't want to be at max pressure when it seems the best accuracy is usually with a combination that is less than max?

 

[Fiftydriver]

If I understand the experiment correctly from your discription they took a set of standard loads and fired it in a chamber with a lead/throat design that would accept up to a 3.000" oal of a 300 WSM.

If this is correct, it would seem to me that they were experiencing a situation such as you would have with a Wby rifle with freebore.

In the starting length, 2.880", you would have at least 0.120" of freebore before the bullet engaged the rifling. If this load was loaded to the top SAAMI pressure loading with this throat design and length, it makes perfect sence.

The freebore is actually a pressure cushion if you will. It allows the bullet to gain linear velocity before it is engraved by the rifling. THis velocity gain results in the bullet conforming to the dimensions of the rifled bore easier if you will then if there was no free bore.

Basically the bullet has gained momentum before it hits the origins of the lands and thus is driven into the rifling easier.

When you shorten the length of this freebore but do not change the load, all you do is lower the linear velocity the bullet gains before reaching the lands, thus resulting in more pressure build up when the bullet does get the to the lands.

In the third length of 3.000". The load is basically to the rifling I assume and as such pressure will certainly increase.

This would be like developing a max load in a given rifle with the bullet seated 0.020" off the lands. Then you decide to seat the bullets so they are 0.020" into the lands when a round is chambered, more then likely you will get a loosened primer pocket.

Basically the same idea is going on here.

Point being, never increase the length of your max loads without decreasing your powder charge and working up the load again. With some hard bullet designs this can be very exciting to say the least.

To make a more informative test, Nosler should have had a 300 WSM chambered with a lead/throat to accept a 2.880" OAL round with 0.020" jump to the lands, then develop the load until they reached 60,000 psi, recorded the velocity data.

Then throated the chamber to accept the 2.900" OAL with the same jump to the lands. Redevelope the load to 60,000 psi and record this data.

And finally lengthen the throat to accept the 3.000" OAL and develope this load to 60,000 psi and record the data.

This data would have been much more meaningful.

This is the reason why if you take Wby factory ammo and shoot it in a short throated rifle, generally you will get into pressure problems. Same reason, load developed for a certain length of throat and when that is shortened, pressure will increase and dramatically at times.

Don;t know it I explained anything or confused the issue more /ubbthreads/images/graemlins/grin.gif!

Good Shooting!!

Kirby Allen(50)

 

[chris matthews]

I am shooting a 6.5 WSM with a 30" at 3150 fps....it's wearing an 8 twsit barrel- if I went with the 9 twist I probably could get 3200ish.... So I think your 24" is a bit short to get the velocities you want. A 7 RSAUM necked down to 6.5 is more effecient than the WSM case and reaches 3200 with a 28" barrel.

 

[goodgrouper]

I agree with Chris.
You might be able to get 3100 from a 24" barrel if you want to have one time use brass. /ubbthreads/images/graemlins/wink.gif

I am pushing 140 bergers at 3100 from a 30 1/2" barrel in my six-five.

The original 264 Win mag would do what you ask from a 24" but at the cost of bad barrel life.

 

[Chester Evans]

Kirby, Chris, Goodgrouper,

Thanks for the feedback. I guess my specific questions are -

1. By designing the chamber so the bullets can be seated even with the neck (increasing powder capacity) do you actually gain added velocity with the WSM case (or the 6.5-.284) or is it limited by SAAMI pressure as the article said with standard powder charges. I used the WSM case only as an example since there was a recent article on it.

2. My first hope was to build a 6.5-.284 with a 24"barrel, short action (2.92" OAL) pushing a 142 SMK 3000 fps. I would prefer the 6.5-284 since good brass is readily available. Based on what I am hearing I am dreaming. What should I expect from this combination with a 24" and 26" barrel keeping the OAL at 2.92"?

3. Based on Chris's idea of a necked down RSAUM, it looks like I could easily get 3000 fps with a 24" barrel. Is this true?

4. My questions on the RSAUM would be - Since no one makes match brass - how good is the Remington brass? Are they going to keep making the RSAUM brass? Obviously I am not an expert in this area, but everyone in this neck of the woods is shooting WSM not RSAUM.

Thanks for taking the time to answer these questions. Also – what lead times (approximate) are you guys running to build a rifle these days? 6-8 months or are you out past a year?

 

[winmagman]

[ QUOTE ]
Point being, never increase the length of your max loads without decreasing your powder charge and working up the load again. With some hard bullet designs this can be very exciting to say the least.


[/ QUOTE ]
Kirby
That's a very timely peice of information. I'm working on loads for a 7 SAUM right now and am seating to fit the mag. which leaves me .095 off the lands. If these don't pan out I'll be seating them longer, Since I'm still learning as a reloader you might have just saved me a little "Excitement"

TacShot
3000 fps with a 142 in a 6.5-284 with a 24-26 inch barrel is probably optimistic. I've got one with a 26" tube and can only get 2980 with a 140 gr SMK and that's a MAX load.(as in don't shoot in hot weather)

Chris