Wednesday, December 19, 2018

Mapping out the DPMS Oracle with low engery rounds

My friend came over with his bone-stock, DPMS Oracle. The Oracle is DPMS's entry level AR. It has a whopping forty rounds on the clock.

The rifle was at room temperature at the start of the test. It took all of five minutes to run so temperature was probably not a variable.

The 55 grain loads with 22 grains of H-4895 all cycled fine.

Two-of-five rounds with 20 grains of H-4895 failed to feed. The bolt stripped them out of the magazine but they did not enter the chamber.

Any statistician will tell you that we need to fire a bunch more rounds to map its performance. This experiment told us where to look.

Practical people will accept that even a little bit of data is better than wild-ass guessing. So, until more testing is done any mods must be 100% reliable at 22 grains of H-4895 with a 55 grain bullet if he wants to retain the "factory" performance.

The Hodgdon reloading site lists 26.0 grains as the top end for one brand of 55 grain soft-point bullets. That suggests a range of 22gr-to-26gr of H-4895 or from 86%-to-100% where 26gr is 100%.

5 comments:

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  2. This may be a dumb question but why are you downloading this round. I don't see the cost of powder and recoil as a reason. Lower velocity will affect down range performance including bullet expansion. So why?

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  3. I have to say, I agree with Mr. Brewi. People often do this type of testing with the Garand to reduce recoil to acceptable levels for kids, wives, etc. Are you just trying to find the lower powder limit so you don't accidentally load a bunch of rounds that aren't going to cycle?

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    1. Hello Mr Brewi and Coast:

      One advantage of buying a complete package from a reputable assembler of ARs is that the components will all play nice together and there will be ample "margin" for less than optimal ammo, lubrication or other non-optimal conditions.

      That can all fly out the window as the enthusiastic AR owner starts playing Barbie Doll and accessorizes it.

      One of the items on my friend's wish list is a solid butt-stock with a "rifle" length and mass buffer assembly.

      There is no guarantee that the new butt-stock won't eat up some of that "margin". The only way to know is to document the rifle as it came from the DPMS factory and then replicate after the Billy-bob job.

      The intention is not to run the low power ammo as a steady diet. The intention is to find out where the current system coughs up a hairball.

      Some of my thinking is explained here: http://eatonrapidsjoe.blogspot.com/2013/08/robust-processes.html

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    2. Howdy Joe,

      In my experience, the rifle length buffer tube and buffer improves reliability when compared to the carbine length. I suspect you and your friend will be pleasantly surprised. If you end up with a different experience when the rifle length buffer and tube is on there, I'll be the surprised one. I look forward to reading more.

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