JP Enterprises low mass .308 bolt/carrier

[E5296]

Newbie here. I am in the process of finishing a semi auto .308. DPMS upper/lower, bolt/carrier; Krieger fluted custom 20" 1/12 barrel, VTAC forearm, Harrris bipod, Timney trigger, Magpul PRS, etc.
My goal is to make this rifle as acturate as possible, will the JP Enterprises PSJPBC-4 .308 Low Mass Carrier/bolt combo make a difference? There doesn't seem to be any other options for "high end" bolt carriers/bolts for this platform that I have found. This thing is about $450.00 so I wanted to find out if anyone thought it was worth it.

 

[ORD]

Re: JP Enterprises low mass .308 bolt/carrier

First, welcome aboard. There is a lot of excellent information to be had here on this forum and I hope you find it useful.

As for your specific questions on the JP low mass carrier setup for the DPMS LR-308 type rifles:

Inherent accuracy benefits...no.

Allowing you to "perceive" less recoil and potentially getting you back on target quicker for follow-up shots and therefore making YOU more accurate...maybe.

Is it worth the additional $150 over and above the $300 DPMS carrier or $100 more than the $350 Fulton carrier...no, not in my opinion (for what that's worth).

The other thing to consider, depending on which DPMS upper you have selected, is that with the JP 308 carrier assembly, you lose the forward assist ability as their carrier is not serrated for the FA to work properly.

Sounds like a nice build-out so far...look forward to seeing/hearing more from you.

 

[E5296]

Re: JP Enterprises low mass .308 bolt/carrier

ORD-Thanks for the insight. I will post pics when Im done.

 

[Casey Simpson]

Re: JP Enterprises low mass .308 bolt/carrier

High speed camera footage may reveal clues to the relationship between bullet exit timing and bolt carrier group movement but since we don't have that right now I feel like my Lr Panther will be more accurate the less it moves about during bullet time in the barrel. A heavier as opposed to lighter bcg, along with the right buffer device, buffer spring, propellant, bullet, and shooter mounting of the gun, seems more accurate to me. Some of this stuff is just gadgetry.

 

[DT1]

Re: JP Enterprises low mass .308 bolt/carrier

I've got a JP low mass bolt carrier on my AR-15 and it does make a noticable difference in felt recoil.

Given the greater bolt carrier mass in the AR-10, it's easy to see how a low mass carrier would go a long way toward smoothing out recoil.

I have two AR-10's and installed JP adjustable gas blocks on both to reduce the effect of the bolt slamming back and forth. A low mass carrier in conjunction with an adjustable gas block would be pretty sweet in an AR-10.

None of these things in and of themselves will positively affect a rifle's accuracy. I've found however, that by reducing recoil and bolt slam, I can shoot both the AR-15 and AR-10 a little more accurately (helps me, not the rifle )

 

[pagana]

Re: JP Enterprises low mass .308 bolt/carrier

I basically built a similar rifle except for .260 and used the standard carrier (Fulton part is from DPMS and is who I purchased from since they were hard to come by a year ago (see Midway for a break on DPMS parts)) with a carrier weight and a flat CS spring. I also EDM'ed .015" from the back of the lug opposite the extractor (Armalite tech paper explains why). If you read Zediker's 2 books on the AR, he explains that the large platforms want to tear themselves apart upon firing and keeping them locked as long as possible helps (along with a host of other operations). This means as much mass, strongest spring, least gas needed to operate reliably = best accuracy. JP rifles are nice, but they approach the build from the perspective of action shooting not ultimate accuracy. having measured these parts to the tenth, I am not impressed with the carriers from DPMS (though they do not actually make all the parts in house). SI Defense is another option for parts for these rifles but I have not tried there wares though they are advertised as being top notch (their price certainly indicates as much). The included picture shows my build for comparison.

 

[Dangerousdan]

Re: JP Enterprises low mass .308 bolt/carrier

Newbe here, Question. I'm looking at a lot of builds here on the board. It seems the choice of build (308) is DPMS and not Armalite? Is it cost or quality. I don't mean to steal the topic
Thanks Dan

 

[Casey Simpson]

Re: JP Enterprises low mass .308 bolt/carrier

Right on, Tool! David Tubb believes the heavier bcg is good too. What does he know? More than me, and he knew enough to win Camp Perry with a big chassis AR how many times? Who in this discussion on this thread has done that? Not me.

Tool is right; Zediker built a 6XC on the lr-308. His smith used several methods to delay breach opening. The result was a very accurate big chassis rifle. Lee Wells built my 6.5x47 barrel. He positioned the port farther out to remedy over-gassing. But hey, don't believe us. Reed the buk.

 

[pagana]

Re: JP Enterprises low mass .308 bolt/carrier

DPMS is likely chosen based on cost, but for me that translates into value. You can certainly spend more, but unless you are actually going into harms way with the platform, you get alot of performance for the money out of a DPMS, at least if that performance is measured in terms of accuracy. The base upper for which I built the dedicated upper shown above was 3/4 - 1 moa accurate with high end factory (Federal Gold Medal and Black Hills red) and 1/2 - 5/8 moa to 500 yards with handloads. With a purpose built upper it is way on better than that. Would either the base rifle or the purpose built upper survive a true tactical environment? No, but then that is what the $2500 and up guns are about. BTW a purpose built top like the one shown or built in Zediker's book is even more expensive than a REPR, POF, etc... but the parts are more readily had.

 

[Hellbender]

Re: JP Enterprises low mass .308 bolt/carrier

Zediker did NOT add BC mass to his 6XC, if you notice. He discusses some pros and cons of the heavier parts (basicly good going back, bad going forward).

He did gas system mods (which I feel is a much better way to go) and a lot of tuning and slicking up parts.

 

[pagana]

Re: JP Enterprises low mass .308 bolt/carrier

You are absolutely correct about the carrier weight. I too moved the port forward and incrementally opened the port for reliable function. In order to achieve the same performance as was had from my bolt gun and not have ejector marks on my cases required the use of a carrier weight. Since I only use this rifle in static positions, I am not as concerned about the feel of the rifle in cycling or its obese weight (19+ lbs.). I built it to see what the platform was capable of if accuracy were the paramount objective.

 

[Hellbender]

Re: JP Enterprises low mass .308 bolt/carrier

Toolmakr....

What size port did you end up with? I had my barrel maker put in an .070 hole to start, I'll open from there.

With the load I am looking to run, I'm looking at approx. 20K psi port pressure (according to Quickload) with my 2" forward port.

Dwell time affects this also, I will have approx 9" of barrel past the port (24" bbl length). IE, longer barrel means longer "push time" for the gas pressure.

I'm going for a lighter field gun, I'm attempting a 12.5 lb OA weight with a S&B 5-25 on top. Lighter barrel, carbon fiber FF tube, etc, etc.

To the OP, don't mean to hijack your thread, my apologies...

 

[pagana]

Re: JP Enterprises low mass .308 bolt/carrier

The barrel is a 27" Krieger with an .085" port moved forward 3" (15" total)in .260 Rem. I started with an .072" and on to .078" but it would not cycle with or without weights. I am shooting a stout but sane load of 123 SMKs with 45.5 gr of H4350 in necked up and turned .243 Win cases. I have not chronographed the semi yet but it makes 3050 fps in my bolt gun with a 30" tube. All I can add is that if I were to do it again I would stay at .085" but move my port back 1" as right now my adjustable gas block is wide open and my cases fall about 2 feet from the rifle (nothing to regulate with my mechanical dwell and gas dwell). Note also that I made this gas tube myself and it has a smaller orifice than OEM. Point is, do what you said above - give it gas and use an adjustable block to throttle the excess.

 

[Hellbender]

Re: JP Enterprises low mass .308 bolt/carrier

I'm (totally) guessing I'll end up around .089-.093", I'll actually go a "little" larger than I need, so I can have a little adjustment range.

I'm gonna play with shortening recoil springs, also, along with my lighter BCG and buffer.