Ideal buffer and spring combo for GMR-13s and 147 grain bullets

[Nemesis Lead]

The title just about says it all.  For a JP GMR-13, what is the ideal buffer and spring combo for GMR-13s running 147 grain bullets?  This can be on a JP SCS system or on independent buffers / springs.  Oh--and I do have a carbine length buffer tube which I am thinking of swapping out for a rifle length.

 

My new GMR-13 runs OK (not perfectly, but OK), but dot bounce is surprisingly high.  

 

I could load lighter and faster bullets, but I have lots and lots of 9mm 147 grain bullets loaded (Production shooter) and want to optimize my carbine for these heavy, slow bullets.  After I exhaust these bullets, I can go to fast and light and a different buffer / spring configuration. 

 

Many thanks in advance!

[Aircooled6racer]

Hello: Buy the JP 223 spring kit. Also buy 2 steel weights and play with them until you find what you like. I am currently running one steel, 2 tungsten and one aluminum weight with the green spring on one PCC. The other I have 3 steel and 1 tungsten weights with the black spring. This is on one Quarter Circle 10 Glock lower and a Quarter Circle 10 Colt lower. Both I am using 147 grain Acme bullets with Tite Group powder. Saying all this I ran the stock JP 9mm buffer setup in the last match in my QC10 Colt lower and did very well with it. You will have to see what works best for you and use the timer as your guide. Thanks, Eric

[Nick S]

My GMR13 runs Freedom Munitions 147s great as it came from the factory. SCS. It's the softest shooting load I have found. No reliability problems.

[Darqusoull13]

A couple of other quick questions. What barrel length and compensator are you running? The different JP comps (and various others) perform very differently at different barrel lengths with different powder charges and types. You absolutely CAN get a comp on a PCC to work though it is not nearly as simple as Open and light/fast bullets are not always the answer.

 

What powder are you using? I'd assume a fast powder like Bullseye or Titegroup if you're using a 147 gr bullet Production load. I'd resist the temptation to load slow powders simply because of how filthy they run in blowback guns. Also as you said, keeping one load between Production and PCC is very convenient. 

 

What path is the dot taking? Is the dot moving up and to the right or is the dot moving up and then dipping down?

 

Given the constraint of your stash of bullets my first suggest would be to pick up the springs pack to tinker with dot bounce. If you can't get dot bounce down with springs alone, then start swapping the weights. I'd avoid swapping to the rifle length buffer tube. There's no competitive advantage to a rifle length tube in 9mm or really with the SCS in general. Plus with the carbine tube you can adjust the stock and get all your friends addicted (you're doing that right?) Last I'd swap the 3 port comp on if you're running the JP Tac comp if this is solely a game gun. The Tac Comp does perform and isn't nearly as loud as the 3 port but it is optimized for another purpose where the 3 port is optimized for games.

Then there's always what I did initially which was load 147's for Production and lighter bullets (124's or 135's) for the PCC using the same powder charge. You might ask a couple buddies to swap 10-20 bullets to see if the dot tracks better this way. And if all that fails, just grip the shit out of it 

[Nemesis Lead]

Hi DS,

 

Thank you for your post.

 

I am running a 14.5 inch (pinned) barrel with the new JP 3 port compensator (not the smaller tactical one).

 

I have TWO loads for the carbine:

 

1)  My production load--147 grain FMJ bullet at 1.10 OAL with 3.6 grains of N320.  Federal or Winchester primers.  I have many thousands of these loaded.

 

2)  A load I made for the carbine figuring that my production load would be too hot (I don't have many of these)--147 grain ACME bullet over an FMJ bullet at 1.10 OAL with 3.2 grains of N320.

 

3)  Dot is moving up and right.  More right than up.  I am right handed.

 

 

Edited May 18, 2017 by Nemesis Lead

[Aircooled6racer]

21 minutes ago, Nemesis Lead said:

Hi DS,

 

Thank you for your post.

 

I am running a 14.5 inch (pinned) barrel with the new JP 3 port compensator (not the smaller tactical one).

 

I have TWO loads for the carbine:

 

1)  My production load--147 grain FMJ bullet at 1.10 OAL with 3.6 grains of N320.  Federal or Winchester primers.  I have many thousands of these loaded.

 

2)  A load I made for the carbine figuring that my production load would be too hot (I don't have many of these)--147 grain ACME bullet over an FMJ bullet at 1.10 OAL with 3.2 grains of N320.

 

3)  Dot is moving up and right.  More right than up.  I am right handed.

 

 

Hello: You can back down that load even more if you are using 147 Acme bullets to about 2.7-2.9 grains. Try the 3 steel weights and the 1 tungsten with the green or black spring. That will flatten it out. Thanks, Eric

[Darqusoull13]

You're in the range of powder charge that I found wasn't quite enough to keep the muzzle flat but was too much for most bullet weights, especially 147 gr bullet loaded so short. I'd think your Power Factor would be around 150+ with 3.6 grains of N320 and a 147 FMJ. (Side note, make sure you keep the lead from building up in the comp with those FMJ's). With the 147 ACME's and 3.2 grains of N320 you will still probably be above 140 PF, just guessing based on my results with my JP 14.5" barrel+JP 3 Port Comp and either TG or BE.  From my notes, between 3.2-4.0 grains of BE had "erratic" dot movement, multi-direction and often did not return to the same point. 

 

Putting in a stronger spring and heavier weight might tame your load down enough. Dropping the load to ~2.7 grains as Eric suggested will also reduce dot movement and that was a load that had a very light horizontal recoil for me. You would just have to separate your two loads. Swapping to a different profile 124 will feel lighter but again, in that 3.2-4.0 range of powder charges with a similar powder I noted erratic dot movement with every bullet weight I tested using the 14.5" barrel / JP 3 port comp. 

I really like lighter bullets with more than 4 grains of BE though 

[Aircooled6racer]

Hello: The stock spring in the buffer is pretty heavy and is used for 308 loads. It is in the 15 1/2lb range. The springs I am using are in the 12.5-14lb range. The stock buffer setup is in the 6.2oz range where mine is in the 4.2-4.9oz range just to give you an idea. The best PCC shooter here is using 165 grain bullets at 130PF to give you an idea. The other guys are mostly using 147's. This is for USPSA pistol matches. Thanks, Eric

[Nemesis Lead]

On 5/14/2017 at 3:41 PM, Aircooled6racer said:

Hello: Buy the JP 223 spring kit. Also buy 2 steel weights and play with them until you find what you like. I am currently running one steel, 2 tungsten and one aluminum weight with the green spring on one PCC. The other I have 3 steel and 1 tungsten weights with the black spring. This is on one Quarter Circle 10 Glock lower and a Quarter Circle 10 Colt lower. Both I am using 147 grain Acme bullets with Tite Group powder. Saying all this I ran the stock JP 9mm buffer setup in the last match in my QC10 Colt lower and did very well with it. You will have to see what works best for you and use the timer as your guide. Thanks, Eric

Hi Eric--I am at work now and cannot look but......what configuration (weights and springs) does the stock JP SCS come with in GMRs?

[Scootertheshooter]

My factory JP runs everything. If you want soft 147g is great. I didn't like how the bolt cycled . Wasn't fast enough so I went with a 124g. Little more punch but feels better

[MHitchcock]

On ‎5‎/‎18‎/‎2017 at 1:48 PM, Aircooled6racer said:

Hello: The stock spring in the buffer is pretty heavy and is used for 308 loads. It is in the 15 1/2lb range. The springs I am using are in the 12.5-14lb range. The stock buffer setup is in the 6.2oz range where mine is in the 4.2-4.9oz range just to give you an idea. The best PCC shooter here is using 165 grain bullets at 130PF to give you an idea. The other guys are mostly using 147's. This is for USPSA pistol matches. Thanks, Eric

 

Any idea what powder/load/OAL he is using with the 165s? I got some 165s to play with a little while ago, but my PCC just showed up yesterday so I haven't had time to tinker.

 

I wonder how accurate the 165s are at 100 yds at 130pf. If accuracy at range is acceptable, would be a win-win for me. Looking to use one load for PCC and 3-gun (on matches I decide to use the PCC). 

[MegaCleanCab]

On 5/18/2017 at 4:05 PM, Scootertheshooter said:

My factory JP runs everything. If you want soft 147g is great. I didn't like how the bolt cycled . Wasn't fast enough so I went with a 124g. Little more punch but feels better

I did this as well....  Also the MBX Comp is awesome.