Reloading for an AR - cases sealing in the chamber

[BobLeeSwagger223]

I bought a Rock River Predator Pursuit 20" and am working on finding a load for it. I would like to use 50gr v-max and Benchmark since it will mainly be a prairie dog rifle and I have tons of components. So far I have tried 25 gr all the way up to 25.8 gr and the cases and the cases come out pretty dirty after they are fired using FL sized LC brass.. A friend told me shooting a hotter load in the rifle makes the cases seal in the chamber better. I did load some rounds all the way up to the book max of 26.5 and am waiting for a decent day to test them. Ultimately accuracy will decide what load I use but I am just wondering if getting the cases to seal in the chamber or the brass coming out dirty even matters. I realize it is a gas gun and I will tumble the brass anyway. I have loaded thousands of rounds for 223 bolt rifles but I am not as familiar with ARs and am interested in learning more. Thanks in advance for any help

 

[jonbearman]

When the case is completely sooted it is indicative of too low of pressure to seal the breach so to speak.Does the book call for a magnum primer by chance.Your chamber being nato may be liberal in size which could also show no signs of pressure and work up slow.I use ball powder in my gas guns and have never had an issue.What kind of scale are you using for the powder as it could be off.

 

[BobLeeSwagger223]

Thanks for the reply Jon! I am using a chargemaster dispenser so I know that isn't the issue. The chamber is a Wylde and I am under the impression it is a little tighter than a standard chamber. I use the Hornady and Sierra loading manuals and they both call for a standard small rifle primer. I use Federal 205 match primers in all of my bolt rifles and I have close to 10k of them so I would hate to switch primers in this current market.

 

[rcw3]

You are using powder with a fast burn rate and light bullets in an AR-15 that has a gas system that was set up for powders with a different pressure curve. The cases are getting sooted up because the pressure curve is dropping off very fast by the time the gas system is cycling and extracting the spent case from the chamber and when the pressures drop off fast and get low it leaves a lot of soot behind - it's that simple. Putting more powder in does not typically help your pressure curve if you use the same powder, the sooting happens when the case is pulled out of the chamber and there is residual pressure in the bore (i.e. all AR's soot cases, it's just a matter of how much).

Robert

 

[BobLeeSwagger223]

Thanks RCW3! I was wondering if powder had something to do with it. I have an 8lb container or Benchmark I had planning on using. Is there any increased wear or damage that could be done with continuing to use Benchmark since it does change the pressure curve and timing of the rifle locking/unlocking?? I am not bothered by the dirty cases I just had the feeling something was off.

 

[vtmarmot]

Powder AND bullets have something to do with it. Slower powder AND heavier bullets will make less soot.

 

[bigedp51]

There is nothing wrong with using Benchmark powder and 50 grain bullets in your AR15 Rifle. You have a Wylde chamber with a throat longer than the standard AR rifle and the chamber pressure is low. Increase your load and remember your throat is longer than the one used for the load data below in the Sierra manual. The Sierra manual lists bullets as light as 40 grains using Benchmark powder and under gassing isn't your problem.

How the AR-15 Direct Impingement gas system works
http://ar15barrels.com/prod/operation.shtml

HOLLIGER ON .223/5.56 CHAMBERS
http://www.radomski.us/njhp/cart_tech.htm

Gun Review: ArmaLite National Match M-15 A2 (Part 2)
http://www.thetruthaboutguns.com/2011/04/foghorn/gun-review-armalite-national-match-m-15-a2-part-2/

 

[mac86951]

I see the questions have been answered already.
Yes more pressure seals the case necks (in a bolt gun). In a Semi-auto there is a balance between lock and unlock, which has already been mentioned. There ARE ways to adjust this on your AR if you are so inclined.

An adjustable Gas Block gives the best granularity, but remember to do load workup before and after you adjust it.
If you are using a Carbine buffer tube, check out Heavybuffer.com for various options (as well as other sites). This
isn't as big of a change and is much harder to fine tune but is usually cheaper and/or easier to swap than the gas block.
TUBB has a BCG weight that simulates a heavy buffer too, but same as above, tuning is really just ON or OFF.

That said, I have a barrel for heavy bullets, and I learned that my upper is a bit over gassed. I haven't done the adjustable stuff yet, instead I've checked different powders. Varget/N140/AR-Comp all left soot, while CFE223/H335 did not. With my heavy bullets and slow powder in my over gassed rifle length system, the pressure curve was broader while bullet traveled down barrel, meaning I was unlocking early. CFE223 gave a steeper pressure spike, but burned faster meaning I didn't unlock sooner. From my experience, take the following: Try different powders, either slower OR faster and see what your rifle likes. Extra soot won't be an issue however, so go ahead and shoot out your Benchmark.

-Mac

 

[Shooterman]

BobLee,
How does your Predator Pursuit shoot the 50Vmax's? I have an older Predator Pursuit 1:8 twist. The lightest I have shot were Sierra 55Gr SP and HP. It shot them well, ~1" @100yards. It is amazingly accurate with the Sierra 69HPBTM. I settled on the Sierra 63 SP due to cost, shoots them .7-1".

My fired cases are very dirty. I primarily use Varget and H335. Good luck with the rifle, I couldn't be happier with mint.

 

[BobLeeSwagger223]

Shooterman: I have only put 160 rounds or so through the rifle. I have test loads ready and am waiting for a decent day to head to the range. Everything I have tried so far has been less than an inch with a few groups around half inch. The barrel cleans up very easily, it should be a promising rifle.

Thanks a ton to everyone for the replies!

 

[moorepower]

I am not a fan of Fed 205's in an AR. Drop your bolt and see if you can see a primer strike. The 205's are a little/lot more sensitive to pin strike than a CCI 450, 7 1/2 Rem or Wolf sr magnum primer.

 

[bigedp51]

I am not a fan of Fed 205's in an AR. Drop your bolt and see if you can see a primer strike. The 205's are a little/lot more sensitive to pin strike than a CCI 450, 7 1/2 Rem or Wolf sr magnum primer.

Any of the small rifle primers that have a .025 cup thickness are fine, the first Remington ammunition loaded for the first M16 rifles had Remington 6 1/2 primers. These 6 1/2 primers would sometimes slam fire if a single round was loaded without the magazine in place to slow down the bolt. The firing pin was lightened and the primers were switched to the 7 1/2 primers to solve this problem.

 

[broncman]

Mac nailed it....add weight to the carrier to extend lock time. An overgassed gun will jerk the case out of the chamber and let gas flow back over the case.

Never had excellent luck with Benchmark in AR's. 3/4" compared to Vargets 1/2" groups.