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Ammo for JP GMR 15


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Yesterday I had a chance to test accuracy of my GMR 15 off of bench rest, with the reloads I use for pistols: range brass, WSP primers, N320 at just over 3 grains, .356 Blue Bullets at just below 150 grains (marketed as 147). The results were more than disappointing at 50 yards. Rounds were all over the place (easily 4-6 inches) so groups were really not groups. I tried a box of 115 Federal factory ammo and it worked much better (about half the group size), but the groups were still pretty large, not something I would expect from a rifle. The rifle was in a sled, not completely immobilized, but stable enough to the point where the dot didn't move from or within a very small black center that was barely larger than the dot itself. 

 

Clearly a bullet that looks like a slug and at less than half the velocity of proper rifle rounds will never travel as consistently as a boat-tail, but then again 50 yards is not all that far. Just the fact that factory ammo shot much better is an indicator that I should probably use lighter bullets at higher velocity, or even just bump up the velocity of 150-s for a higher PF. I'd like to save myself some time and narrow down the experimentation to something likely to produce good results.

 

If you went systematically about evaluation of your GMR (or any other PCC) performance, what are the general trends you observed? Any combination of fast/slow powders, bullet weight, bullet shape, etc. that tend to get better results? Any issues with the blowback design that limit what and how to reload? 

 

I am primarily in USPSA, but I know that a lot of shooters use PCC-s in matches where longer distance is required and where performance needs to be optimized before it makes sense to shoot a PCC. Any input would help. 

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I second what davsco said above about a magnified optic just for testing purposes. It doesn’t take much at PCC distances, so even a 4 power scope works well but the higher the better, within reason of course. Also as much as I love coated bullets I cannot get them to shoot groups at distance out of my two PCC. Throw a plated bullet or a cheap jacketed round nose in either of them and groups at 50 yards shrink from 6+ Inches to 3-4 inches. Go even farther to a good JHP such as a Hornady XTP and they will cut 1-2” at 50 yards. Velocity hasn’t seemed to affect groups that much and these tests were out of a cheap 16” barrel and a Taccom ULW barrel. 

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1 hour ago, grsdoug said:

I run 124gr Precision Delta JHPs with 4.0 grains Titegroup @1.10 OAL. Getting sub inch groups at 50. Top load in all my pcc’s, MPX, Scorpion, Nordic, Stern, and GMR.

I running the same bullet with 3.8 grains Titegroup @ 1.12 in a GMR-13 and getting good accuracy and no leading in the comp.

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My GMR13 will shoot very accurately with good

quality jacketed bullets but only mediocre with coated or plated. Have seen the same results with other people's pcc also. I don't think the powder selection is as important as bullet selection. 

 

 

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Thanks guys - this reminds me that I actually have a decent amount of 124/.355 Montana Gold JHP bullets that I use for 38 Super Comp. I'll go ahead and load a batch of 9mm and test the accuracy. 

 

Since I have to tinker with the load now (must add about half a grain compared to coated), I'll start with a recipe I tested with 130 MG FMJ. As a side issue, is there any noticeable increase in accuracy at some specific muzzle velocity, or should I just go for the consistent PF, e.g., around 130? 

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22 hours ago, Tampa-XD45 said:

Which barrel are you using?

The JP 14" (I believe) - it's the one that has to have the brake pinned and welded to meat OAL. It is NOT the pistol barrel with the shroud, if that's what you're asking. 

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22 hours ago, davsco said:

what magnification scope are you using?  just a non-magnified red dot?  if so, do you have a magnified scope you could throw on?

Just a red dot (Holosun 510C green; I guess it makes it a "green dot" these days...). 

 

I do have a bunch of scopes I could use, but I don't want to go that route. Yet. The groups were way too big for the dot so there is no doubt that I have to sort out the load first. If I get to the point where I'm getting decent groups, it sounds like a very good idea to throw on a scope and measure the accuracy accurately (say that three times fast). I'll see how I do with the JHP bullets and if it indeed gets close to being good, I'll probably try with the scope.

 

It's just that I don't want to have to re-zero the dot - I guess it's no big deal and I shouldn't be lazy... :)

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maybe also double check your optic.  is it on tight and centered.  if you have elevation and windage locks are they tight.  all i did was sight mine in at 25 yds and went shooting at matches, but i don't remember even thinking twice about any lack of accuracy.  i will try to check with a variety of factory ammo (i don't reload) and will get back to you.

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