questions on odd cyclic issues

[Shaggy151]

I'm fairly new to the world of 6.5 Grendel, but I'm pretty familiar to building and maintain AR's in general. So,....here's my issue. I have been trying out various handloads to see which powder/bullet combination works and functions best in my rifle, and lately i've encountered some feeding issues.

OK,...i'll give a lil details on my 6.5 Grendel upper. Its a Bear Creek Arsenal 20" stainless heavy barrel 5R rifling, rifle length gas system, standard .750 gas block, stainless rifle length gas tube, 15" M-Lok free float handguard. Bear Creek arsenal 6.5 Grendel II BCG, standard charging handle.


The rifle has functioned flawlessly using Factory Hornady and Federal loads, and has performed flawless using most handloads. Also,... my magazine is a C-products and an ELander. I have ran 90gr Speer TNT's and 95gr Hornady V-Max bullets, 120gr Hornady ELD match, and 120gr Nosler Ballistic tips-all using various loads of IMR 8208xbr flawlessly.

Obviously some were much more accurate than others, but last week I tried some 123gr Nosler Custom competition bullets using Vihtavuori N540 using several different 5 round test weights as well and it performed flawlessly and was very accurate all 5 different weights were under 1 moa and the best was .46".

I then tried some 8208xbr loads and I got round after round where the bolt would fail to load the next round and then would also fail to hold open the bolt on the last round. It even jammed while cycling a few times. I switched back to the N540 loads and all was perfect. So today I tried some more 123gr Nosler CC bullet loads using 8208 xbr, AR-Comp and N540, the 8208 XBR and AR-Comp would fail to pick up the next round pretty regularly and would occasionally jam loading a round.

I then switched to the N540 and it functioned perfectly. I even switched magazines and nothing changed when using 8208xbr or AR-Comp with the 123gr bullets. Also,... the past few weeks it was 46 degrees and today was 66 degrees. Also,.... my gun ejects everything at the 3:30-4:30 O'clock position. Any suggestions or opinions. Its baffling me because ive run both 8208xbr and AR-Comp using different bullet weights and my gun functions flawlessly...

[VASCAR2]

You didn’t mention what buffer length tube, buffer spring and weight of buffer your using with the 20” BCA barrel. Looking at burn rate charts N540 is a slower powder than IMR8208XBR or AR Comp. You didn’t list your charge weights so it is hard to say if your under gassed or over gassed but the bolt speed/unlock time with faster powder could be the issue.

The issue could be how far you bumped the shoulder back or seating depth of the bullets. The ogive of the bullets are different and the 123 grain CC bullets could be into the lands or seated to deep which can affect pressure. I suggest you try dropping your reloads into the chamber and see if the cartridges fall out when you raise the muzzle. If you need to tap the barrel or use a cleaning rod to remove the cartridge your cases are out of spec or bullets are loaded to long. You can use a black marker to color the nose of the bullet to see if your into the lands.

[lazyengineer]

You're short-stroking. Assuming nothing physical like a gas block or key coming a little loose; ARComp is a little infamous in the 223 world of short-stroking rifle-length gas systems, particularly with heavy bullets. I suspect it's tuned more for carbine length gas systems, but that's just a feeling. 8208 also tends to run best at the higher end of charge weights. Over the years, I've learned that powders tend to prefer to run on their hotter end, for best performance - as that is where most are engineered to run, and where they give their more complete burns (and also, least fouling).

I suspect this is a simply a case of short-stroking due to the powders being a touch on the fast end, possibly combined with the charge being a little on the light end. Also bear in mind that one poster (Mr @centerfire ) mentioned yesterday that the Noser 123 CC's he had, experienced some diameter variability. What that can mean, is the bullet tension on a lower diameter bullet might be enough less inside the casing, for the bullet to slip out before what's needed to cycle up to peak pressure build-up, and robbing some of the powder of that initial high pressure for a more complete burn. Indeed, in the past, I've experienced that mild-charge weak loads, run much better if I put a heavy crimp on them; which might be an adaptation of what yours are doing, depending on how light your charge is.

My recommendation going forward? Increase your charges of ARComp and 8208. If you are using a bushing sizer, switch to a smaller diameter bushing to give a smaller diameter neck, and consider eliminating or turning down your expansion ball. I personally don't even run an expansion ball in my sizing die anymore, you usually don't need it and it causes more problems then it solves.

[Shaggy151]

yeah I realize now I was being kinda vague. I'm running a standard A2 rifle length buffer tube, buffer and spring. Charge weights are as follows:
8208xbr= 26.3gr vel 2376fps stdev 15.9, 26.6gr vel 2424 stdev 18.7 , 26.9gr, 27.2gr vel 2348 stdev 10.7, 27.2gr vel 2456 stdev 9.2,.... OAL is 2.260,.... I actually measured it 1.751 to the ogive using the Hornady bullet comparator. It was once fired Hornady brass.
charge weight for the N540 was 25.0gr, vel 2093 stdev 5.5; 25.6gr vel 2169 stdev 11; 26.2gr vel 2226, stdev 11.7; 26.8gr vel 2291, stdev 25.4; 27.4gr vel 2321 stdev 5.3 all were seated the exact 2.260 OAL

[Jeepster18_88]

Unless I'm reading this wrong, the velocity dropped when you went from 26.6 to 26.9 gr of 8208xbr, then jumped back up when you went to 27.2 gr. Something doesn't sound right with those numbers.

[Shaggy151]

Unless I'm reading this wrong, the velocity dropped when you went from 26.6 to 26.9 gr of 8208xbr, then jumped back up when you went to 27.2 gr. Something doesn't sound right with those numbers.

I agree,.... I honestly didn't pay attention to the numbers when I was at the range. That seems pretty impossible, that the velocity went down as the charge went up. I assume its possible that I loaded the wrong charge weight. I checked the chrono app and I wrote down the numbers in the correct order, so,... I assume I charged it with the wrong charge weight.

[grendelnubi]

I have seen the lower velocities with higher charges, mainly using CFE and some 8208 but fewer times with 8208. My issues occured during summer months load development. In my case I believe it was contributed to pressure spikes due to Texas heat.

In your case it appears the 27.2 charge was a higher than expected velocity based on .3 grain increments which would indicate to me it did pressure spike at this load. You may have went past your node at this point. I would consider removing 8208 from your powder choices for this projectile, it may not be worth the effort to get it to shoot.

Regarding the AR Comp short stroking, I am seeing short stroking using AR Comp with 26.9 grains when using Nosler 129gr ABLR's. I have increased my gas block but have not had a chance to test it.

The N450 powder is a slower burning powder than the other powders you are considering which would increase the gas port pressure due to the longer pressure curve. This may be the reason you are not seeing the issues with N450

If you are using an adjustable gas block, increase it for AR Comp and see if it corrects your issue.

[NugginFutz]

Your 26.x grains of XBR is on the light end of the known typical charge weights for this powder and bullet weight. Most people find the upper limit for 8208-XBR to be 28.2 - 28.5 gr. Starting 10% lower puts it right around 25.7, so you're at the low end for the average load of XBR.

I'd double check your charges, and take it into the 27's to see what happens. Just be careful with the 120-123 grain projectiles and XBR. It is one of those powders where reloaders report a steep pressure curve when at or above 28 grains, depending on the individual barrel.

[LRRPF52]

Charge weights are low with 8208XBR as NugginFutz pointed out.

With RLGS, lower charge weights, maybe a slip-fit gas block, and cold weather, those can all combined to not providing sufficient gas volume or pressure back into the carrier.

[Shaggy151]

Well,... I started seeing pressure signs on my cases as well. The AR-Comp really started leaving some pretty good scrapes on the headstamp. I removed my hand guard and did notice there was some signs of slight gas leaking around the gas block. I'm thinking an adjustable good quality clamp on might me what I need. Right now im just running a standard nonadjustable gas block. What powders are you guys using with 120-123gr bullets?

[VASCAR2]

I’ve gotten good accuracy with CFE223 with 123 grain A-Max, 123 grain CC, 120 grain Sierra ProHunters and 120 grain Nosler BT. I also tested Winchester 748 and BLC2 but got better velocity and accuracy with CFE223. One Friend has loaded 120 grain Nosler BT and 123 grain SMK with Accurate 2520 and another Friend uses LeveRevolution with 123 grain SST.

I bought a pound of AR Comp to test but just haven’t had time to do any testing. I have IMR8208XBR but have only tested it with 107 grain SMK.

[Shaggy151]

I loaded up some starting loads of 123gr Nosler CC with some different loads of BL-C(2) and some H335. I decided to try some GM205MAR Federal Gold Medal Match AR primers. I primarily use CCI #41 primers but I think they are causing my pressure to rise a lil quicker than what I feel it should. My best groups were using the standard Gold Medal Match GM205M primers with N540 and Nosler CC bullets, so I expect the GM205MAR primers to be just as good. I hear the accuracy can be pretty good with the BL-C(2) using lower to mid charge weights.