Armory Sponsor
Posted: 10/22/2011 12:32:25 PM EDT
|
Let me preface this by saying I shot 200 rounds of Federal Gold Medal Match 168 BTHP SMK's through this gun a little over a month ago and it was flawless. Sub MOA accuracy, perfect function and never a single sign of pressure. So I am kind of perplexed with my issues.
I just tried my first go around with reloading for my Mega Ar-10 and it didn't go well. All brass is once fired that has been cleaned, sized, decapped, chamfered and deburred. I also checked each case in my Dillon case gauge and all cases looked fine in the gauge. Bullet used: Sierra 168 BTHP MK Powder: Varget Primer: Federal 210 and Remington Large Rifle I started my powder charges at 42 and worked up in .1 increments just to see where the pressure signs started. Right off the bat, I had primers that looked like the ones below. It looks like they "Flowed" back around firing pin. What would cause this? My charge wasn't too high. No flattening of the primers either. 
Here is my bolt face as I have seen some people take pictures after having pierced primers. 
Any help is much appreciated. Also, gratuitous gun picture. 
Mega Monolithic upper and matching lower. 18" Ranier Arms Ultramatch barrel-Mid length gas system DPMS BCG Gieselle Super Dynamic Enhanced Leupold VX-3 8.5-25-50 in a Larue mount 
|
|
Cratered primers can be caused by larger than spec firing pin holes. It's common on M1A's with perfectly good ammo.
Federal brass leaves a lot to be desired when it comes to reloading. Can you get a chronograph reading? You may be over pressure. 42.0 grains is not a light load. Gas guns can not handle the same powder charges that bolt actions can. |
|
Try some thicker or harder cup primers like the CCI#34. That might reduce the crater rim somewhat.
From CCI website: Military-style semi-auto rifles seldom have firing pin retraction springs. If care is not used in assembling ammunition, a “slam-fire” can occur before the bolt locks. The military arsenals accomplish this using different techniques and components—including different primer sensitivity specifications—from their commercial counterparts. CCI makes rifle primers for commercial sale that matches military sensitivity specs |
| Something else to consider- in your first pic, you show both a winchester 308 case and what looks to be a WCC nato case. In 7.62/308, militatry brass does have less internal case capacity than commercial brass. I know in 5.56/223 the difference is small enough that most people dont bother changing load data. Not so with 7.62... I would definately seperate your brass and work up loads seperately. Also, in my 2 AR10s, the best early indicator of pressure is a smear from the ejector on the case head. |
|
I made a more loads today and some different issues this time around. I sorted my brass as well.
Started at 40 GRs of Varget, which is below Sierras minimum. Went in .2 increments to 42 and everything was fine. So i proceeded to move up in .1 increments from there. Everything looked fine all the way up to 42.9 so I just stopped there. I then decided to back down to 42.6 gains of Varget and shot some groups. First shot felt good, but I didn't hear the brass eject. So I checked the chamber and sure enough, the extractor had ripped a little chunk out of the rim and left the brass in the chamber. The head stamp was CBC 308 I believe. I proceeded to shoot the next 9 loads I had and I noticed pronounced marks on the rim of the case from my extractor. Any ideas what could be causing this? |
|
Quoted:
Quoted:
I proceeded to shoot the next 9 loads I had and I noticed pronounced marks on the rim of the case from my extractor. Any ideas what could be causing this? You're still too hot. Too much pressure is causing the ejector marks. Hard extraction is the result of too much pressure and making the brass expand a bit more than normal hence it is sticking in the chamber and the extractor is having a tough time removing the brass. back off the load. let me look for a few pics of hot loads and you will see. Found some pics. 
|
|
If you start to see marks like these on any of your brass, well rifle anyways, it is a hot load and you are running a risk with too much pressure.
Oh and like someone else mentioned. .mil rifle brass and commercial brass have different interior voulmes. So on the .mil brass like LC you have less internal voulme and can run higher pressures compared to the commercial brass. So lower the starting charge by 10% when using .mil brass. About the only exception is the .223/5.56 where case size is about the same between the 2. |
| The simplest way to determine internal capacity is to weigh the different brands of brass after the have been resized and trimmed. Military brass (I think CBC is) usually weighs 178 to 181 grains unprimed and ready to load. Winchester can be as light as 155 grains. Big difference in weight. The walls and head of the mil. brass is much thicker and it needs 2.0 grains less powder to create the same pressure as Winchester. |
|
Quoted:
The simplest way to determine internal capacity is to weigh the different brands of brass after the have been resized and trimmed. Military brass (I think CBC is) usually weighs 178 to 181 grains unprimed and ready to load. Winchester can be as light as 155 grains. Big difference in weight. The walls and head of the mil. brass is much thicker and it needs 2.0 grains less powder to create the same pressure as Winchester. So using Milspec brass, and acheiving the same pressures, will you get the same velocities? Just curious. |
|
Quoted:
Quoted:
The simplest way to determine internal capacity is to weigh the different brands of brass after the have been resized and trimmed. Military brass (I think CBC is) usually weighs 178 to 181 grains unprimed and ready to load. Winchester can be as light as 155 grains. Big difference in weight. The walls and head of the mil. brass is much thicker and it needs 2.0 grains less powder to create the same pressure as Winchester. So using Milspec brass, and acheiving the same pressures, will you get the same velocities? Just curious. All other things being equal, yes. |
|
Quoted: Quoted: The simplest way to determine internal capacity is to weigh the different brands of brass after the have been resized and trimmed. Military brass (I think CBC is) usually weighs 178 to 181 grains unprimed and ready to load. Winchester can be as light as 155 grains. Big difference in weight. The walls and head of the mil. brass is much thicker and it needs 2.0 grains less powder to create the same pressure as Winchester. So using Milspec brass, and acheiving the same pressures, will you get the same velocities? Just curious. Pressure is one factor, and powder burn rate is another. For instance a pistol powder will hit peak pressure very quickly with a much lower charge, than a rifle powder would. Less powder to burn will equal less velocity. |
Armory Sponsor