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Old July 22nd, 2020, 11:13 AM #1
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300 blk shoulder and headspace

I apologize if this has been discussed before but I couldnít turn up much in a search.

I consider myself intermediate in regards to reloading experience. Enough to make safe and reliable reloads as I have had a great mentor. Anyway I have Been getting serious about loading 300 blackout lately. Iím mainly interested in subsonic loads and have amassed about 100 peices of various once fire factory brass.

My current load Iíve been working on is a 200 gr Sierra matchking over 10.7 grains of imr4227 and a cci mag small rifle primers. I also used standard small rifle with only a slight velocity difference. Its moving at a pretty consistent 1030fps out of my 10.5Ē ar pistol. I wasnít really looking at accuracy potential yet as I have been focused mainly on velocity.

So the point I am at right now brings me to asking you guys with case forming experience. I have several thousand pieces of 223/5.56 brass that I have just about wore my poor little midway tumbler out cleaning it up. I have separated all of it into the cases I have researched as having usable neck thicknesses for forming the black out brass.

Sorry for the long back story leading up to where Iím at now but this all leads up to my questions. I got a good system for cutting cases and I have been forming them with my lee sizing die. I use the worlds cheapest trimmer and it works great for putting them right at 1.259Ē The shoulders however do not look as defined as the factory brass. Not exactly sure how to measure the shoulder angle. Does this cartridge headspace off of the shoulder? Thatís what has me concerned. I know the lee die isnít the best option for forming and Iím looking for suggestions on a dedicated forming die. I did fire to of the formed rounds in my ar pistol and they functioned fine. However I have a mgm barreled encore that I had some light primer strikes with factory 300 brass but I think that maybe headspace issues with the rifle itself. I have a headspace testing and shim kit coming for that. Also a heavier hammer spring.

I have never really overly concerned with head spacing on my reloads but I do have a new understanding of its importance. Could you recommend the right cartridge gage I need too?

My whole reason for jumping into this endeavor is the continuing absence of reloading components. I have been building up supplies of h110, lil gun 4227. Also trying to acquire as many 30 cal bullets as I can. Primers have proven to be difficult to find lately too. I was able to aquire a stock of older win 6 1/2 primers and Iím only planning on using them for low pressure sub loads. I also have a line on some #41 milspec primers.

Iím sorry for the long winded post but any help or advice would be much appreciated. I know there are other resources of info but Iím finding out that people can be pretty snobby towards newbies of this round. Also could you let me know if I have left out any critical details?
Thank you
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Old July 22nd, 2020, 11:49 AM #2
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Sheridan cut away case gauge.

https://ballistictools.com/store/she...-slotted-gauge

It will tell you where your case failed if it fails the gauge.
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