Another AR thread

[Harter66]

I'm not new to reloading or even the mountains of cast that I shoot so this makes my question maybe more perplexing.

I usually load autos from a start load (experience says 10-20% of the load window above start) an work up on paper for function then smallest group which is usually not very far above correct function. The AR however has done a strange thing . I have 15 pieces of brass from 1 lot of 1 gr over/under matched brass to work up 3 shot sets . I usually shoot over a Chrony if for no other reason to save components if I NEED a particular velocity for a particular bullet .

In this work up 18.5-22.0 RL 10X with a 140 gr cast bullet , Federal srp brass, CCI 41. What is odd and sent me back the boards was that the lightest loads of course failed to function , #3 of 8 functions ok and just made the hold open . # 4 however only hooked the follower for hold open . The velocity increase was typical with some steps gaining 15 fps and some gaining 25 fps .

My theory is that the #3 load was the fastest of the string and generated just enough to catch and stay on the hold open (tolerance stacking ,mag springs etc) while the # 4 load was slightly more but didn't hit the catch (writing is good for me, different mag ,different springs , just a touch slower and maybe the cause of the aboration.

Quirk of the platform but not exclusive to it I bet ........

Call this an observation as having considered it as I wrote it down it becomes clear that mag variations are the most likely cause .

[Motor]

What caliber AR are you shooting a 140gr cast boolit? 300 ?

Yes when you are on the edge of function just a very small difference can yield different results. This same type of thing happens very often to auto pistol shooters. My sister-in-law has to have a revolver because no matter how hard she tries she can not get a auto loader to function properly due to her grip.

If you were shooting your AR from a solid rest like a "lead sled" you may even get fooled into thinking a particular load works only to find later that shooting it off hand it won't function.

Motor

[Harter66]

6.8 SPCII.

A builder has suggested that I may be walking the ragged edge . A perfect storm was the expression used . It is suggested that the load pressure is being maintained beyond the mid length gas port and that what is happening is that the case isn't releasing fast enough for the full ejection cycle . While this is good for boolits it's not so much for function. I had thought about a slower powder but if the suggestion is correct that will actually make it worse . What I need is a lower pressure load that has a faster rate and a flat peak . No clue where to look for that so the next choice is to change boolits.

[Moonie]

140gr is a tad heavy for 6.8. I've had good luck with 134gr Ideal boolits with 18gr of H4895. It is very possible that smaller charges of RL10x just doesn't produce enough gas volume to function it properly. I've never used RL10x for light loads but I know RL7 works well for it in my 300 AAC with subs.

[Harter66]

I'm actually right in the J window and getting into the top both for charge and speeds.