Bolt carrier mass choices

[thirstman]

I'm building an AR-10 in 6.5CM and I'm using a number of JP parts. I am using a super match barrel, high pressure bolt, silent capture spring, and an adjustable gas block.

My question is what bolt carrier mass and buffer mass profiles should I be considering, and does shooting with a suppressor change this much?

I saw that many folks recommend h2 buffer. And I saw jp website says to pair the vmos(h2) with the h2 buffer. But I also see a lot of folks using the low mass. I'm very confused.

 

[Evintos]

Low mass is something you use to reduce reciprocating recoil impulse (that thump/jarring effect you feel when the BCG and buffer taps the rear of the receiver extension/buffer tube and the forward thump/dip when the BCG returns into battery). To understand it, lock the bolt or slide back to the rear on a pistol or rifle, then hit the bolt release or slide release. You'll feel that dip/thump/jump. That force exists both forwards and backwards in a semi-auto, assuming the slide or BCG doesn't short stroke.

Low mass in large frame ARs necessitates an adjustable gas block. Suppressors add more back pressure (even low backpressure suppressors) and necessitates higher total mass or reduction of gas flow (via an AGB). Depending on the suppressor, this amount can be drastic.

If you want a relatively lower fuss setup, go with standard mass and an H2 + adjustable gas block.

If you like to or don't mind tinkering and understand how the system works overall and know how to do intermediate troubleshooting in field/at the range, low mass can give you some advantages (such as less overall movement allowing you to spot your shots more easily).

The above does not take into account gas port sizing. Excessively large gas port sizes on your barrel can make the "standard" recommendation of full mass and H2 buffer completely moot. Undersized or properly sized gas ports can result in a lower than H2 mass buffer (such as H1, standard). The bold also doesn't take into account perceived reliability issues (debatable) in less than ideal conditions (sand, mud, snow, dirt, fouling) with low mass.

Depending on the adjustment resolution for your AGB (example 18 clicks from full open to close is basically 5.5% per click adjustment values in port size) you may or may not have a fine enough resolution in order to fully optimize the system (whether full mass or low mass) in which case you would then use spare springs (SCS spring kit) and play with mass as well as spring rate to fully get optimal performance.

 

[thirstman]

I appreciate the detailed response Evintos. I definitely want to get hands on and play around to tune it. Do you know if a lighter bolt carrier is going to affect the cases? I heard I should use the high pressure bolt for a number of ammo related factors. I do plan to reload.

 

[Evintos]

The main signs will be extractor (torn rim damage) and ejector marks if the tune is not correct whether low mass or full mass with standard or with a high pressure bolt. Essentially you'll have early unlocking that presents itself as overcharged/overpressure loads.

It's hard to tell whether or not you would have issues with a standard bolt with your loads and barrel combination. Most of the time, proper tuning of the gas system (AGB and mass) eliminates the issues however some users will still experience problems even if the extraction/ejection/gas timing/mass values is correct. This depends on powder burn rate characteristics (and resulting chamber and gas port pressures), how the chamber/throat is cut, and various other factors which would result in things like primer piercing, primer cratering, primer flow into the firing pin channel with a standard bolt.

A "high pressure" bolt has a slightly smaller firing pin hole/channel and corresponding narrower diameter firing pin. Some will advertise improved extractor geometry as well as ejector. This would basically prevent those issues even with proper gassing.

Short answer - if you don't want to have to deal with potential issues even with a properly tuned/timed gas system, go with a high pressure bolt.

Something to consider with a high pressure bolt, the main proprietary parts would be bolt and firing pin. Potentially the extractor and ejector would be proprietary as well (although usually interchangeable with standard parts). Usually the springs, cam pin, gas rings, cotter pin are interchangeable between a standard and a high pressure.

Key important things - the bolt should headspace to the barrel; this is especially important with large frame ARs (while most manufacturers are close, there's no real standardized TDP for large frame ARs and so a few thousandths in difference can result in functional, semi-functional, non-functional, to catastrophic).

The carrier mass doesn't affect headspace, however the carrier length must fit the receiver (older Armalite AR10 length vs the more common large frame AR308/LR308/DPMS 308 length).

 

[longshot2000]

What @Evintos said. Keep it simple. Standard mass BCG. I know there are lots of claims of low mass is blah, blah, blah. try it one day and see what you think. I think adjust the buffer weights up and down using tungsten weights and a DIY standard carbine buffer. Next, adjustable gas flow if you think you need it. I like riflespeed.com if you go that route. Mostly, I avoide adjustable gas, unless you are switchting between suppressed and unsupressed.

 

[Stainless75]

I assembled a JP LRP-07 build kit a few years ago. 6.5 CM, 24 inch Proof Research CF barrel, Hi Pressure bolt (JPEB-308HP), LMOS carrier (JPBC-4SP) and H2 SCS buffer (JPSCS2-10H2). I mostly shoot SRP Starline brass with 140gr ELD-M or Hornady Match 140gr HPBT in this rifle with 41.9gr of H4350. Brass comes out perfect.

A buddy build something similar, but using the JP VMOS carrier and the standard AR10 SCS buffer. His load is the same, but his brass looked worse. We added a Tungsten weight to his standard SCS and this resolved the issue.

I prefer adding weight to rear of the reciprocating mass in a gasser.

 

[thirstman]

Thanks fellas, this has been very informational. I have an adjustable gas block from superlative arms with 30 increments of adjustment in my fit, and the high-pressure bolt that comes with the jp super-match barrel is head-spaced to the barrel.

I think what I got out of this is, start with a standard mass bolt carrier, and play with the SCS springs and weights if the high pressure bolt and gas block adjustments aren't enough to ensure brass integrity.

I feel like if I choose a carrier mass on either end of the spectrum, I may travel too far on the scale and have difficult tuning my rifle using the other components(gas block and buffer). I think I'll pair the standard mass carrier with the H2 SCS then?

Weights for the buffer are cheaper than changing the carrier, and the spring pack comes as a $20 addon for the SCS, can't beat that.

 

[Evintos]

Thanks fellas, this has been very informational. I have an adjustable gas block from superlative arms with 30 increments of adjustment in my fit, and the high-pressure bolt that comes with the jp super-match barrel is head-spaced to the barrel.

I think what I got out of this is, start with a standard mass bolt carrier, and play with the SCS springs and weights if the high pressure bolt and gas block adjustments aren't enough to ensure brass integrity. Yup and you want to increase mass if the gas adjustments aren't enough. If you've increased all the mass you can (all tungsten weights), then go with stronger spring rating.

I feel like if I choose a carrier mass on either end of the spectrum, I may travel too far on the scale and have difficult tuning my rifle using the other components(gas block and buffer). I think I'll pair the standard mass carrier with the H2 SCS then? Good starting point.

Weights for the buffer are cheaper than changing the carrier, and the spring pack comes as a $20 addon for the SCS, can't beat that.

If you ever get the itch and want to go down the rabbit hole of playing around with mass tuning after getting all set up with standard mass carrier with the H2 SCS, switch out one of the tungsten rings on the SCS to a steel one (H2 to H1 weight). Then retune gas block. If you find that the feel is off if you go too far in one direction with mass (like if you prefer between H1 and Standard weight) that's when you play with spring weight. Go in increments until you find that good balance in shooting feel/flatness/recoil impulse characteristic that is comfortable (and reliable).

Muzzle device also plays a big part in the feel. Muzzle brake vs flash hider vs vs compensator vs suppressor will all feel different and you can wind up chasing your tail if you don't make some compromise on the "feel."

Assuming any combination of system mass/spring rate is 100% reliable with no brass issues, what you choose in terms of mass and gas tuning really is subjective.

 

[thirstman]

@Evintos this advice should get me where I need to be, thanks again for the heads up. I'll have to ping you and bump this thread when I finish my build and get my first group.

 

[Malcolm31]

Great responses and info here.

I would also suggest calling the techs at JP and asking for input. They have helped me with all of my JP builds and questions from bench stuff to 3 Gun and PCC.

While you’re there feel free to use CODE: 68SS0717 to save 10% on JP parts or 5% on complete JP rifles.