Increased powder charge = short stroking AR

[SnkBit]

I have been fighting with a new AR purchase that suffers from FTF's. I have narrowed it down to what I believe is a charge weight problem. I ran 23gr of RL-15 behind a 75gr Hornady BTHP and had no problems. I increased the charge weight by .2gr increments and began suffering FTF's around 23.6gr. After that they became more definite up to the max charge tried of 24gr. What would cause the bolt to short cycle due to higher velocity or higher pressures? I plan to load for this rifle around 23.2gr being as that's where it seems to be most accurate so it's not necessarily problematic but this is still a issue that none of my other AR platforms have. I don't want to have problems with this in the field.

 

[ponchair]

Re: Increased powder charge = short stroking AR

Are you blowing out primers or having any over pressure indicators? Have you backed the load down to see if the AR cycles like normal with less powder?

I am not an expert but you might have problems with the bolt cycling due to maybe needing a clean, or the cases are getting tight in the chamber and the BCG is unable to cycle completely.

 

[308sako]

Re: Increased powder charge = short stroking AR

deleted in protest

 

[BCP]

Re: Increased powder charge = short stroking AR

24 is the max load for 223 with RL15 and a 77 according to alliant's site. It could be that the cycling issues are due to extraction problems. Did you notice any pressure signs on the case heads? Shiny marks, imprints from the extractor, pieces of brass on the bolt face?

 

[Pthfndr-CA]

Re: Increased powder charge = short stroking AR

Sounds like the gas key came loose.

 

[SnkBit]

Re: Increased powder charge = short stroking AR

I have already checked for gas leaks and loose gas block. I had another thread when I first bought the rifle trying to figure out why it had the same issues with factory ammo. It isn't damaging the case or really flattening the primers so I wouldn't think the pressure had become that excessive yet. Like I said, I intend to load at 23.2gr where none of these problems existed. I just want to figure out why its doing this so that I can be sure it will not pose a problem later in the field.

 

[Clark]

Re: Increased powder charge = short stroking AR

With 33 gr Vmax moly and Blue Dot, there is no amount of powder that will cycle the action.

At 4200 fps, the pressure is not there for long.

 

[gunsnjeeps]

Re: Increased powder charge = short stroking AR

Could the bolt move faster than the round coming up from the magazine? More of a curiosity question than a solid idea.

 

[BCP]

Re: Increased powder charge = short stroking AR

Weak mag spring and/or the shape of the cartridge (sometimes a really long BTHP won't feed right) can do it.

Personally I think he's just hit his max for that load. I've heard of people going as high as 45 with that powder and bullet but I don't see the point, if it functions fine at 23 don't mess with it.