So my factory new 6.5 creedmoor brass had a cartridge base to shoulder measurement of 1.531 inch. After firing, they measured 1.533-1.538. When I bump the shoulder I feel like I need to go atleast 1.528” to make sure they’re chambering well (ar10). 10thou is a lot of bump I know but I do anneal after every firing. This seem like a good plan?
Shoulder bump for AR10
- Thread starter cmcq
- Start date
RegionRat
Gold $$ Contributor
It sounds a little off?
If it were my rig, I would attempt to take just a few cases and focus several cycles on establishing the sizing recipe. On a fresh upper, these would be the ones I use to boresight and start up the new barrel.
ETA: This is the time to load at the range with portable hand gear. It saves round trips and you go home with solid answers.
Your initial number is lower than your factory value which sounds suspicious. That may mean the chamber isn't happy with your dies for example.
Will a once fired case go back into the chamber without sizing?
I would measure the shoulder junction diameters and the 200 line diameters to see if your chamber and dies are the reason your number is already shorter than the factory rounds you started with. Those diameters may explain why you had to go lower.
If it were my rig, I would attempt to take just a few cases and focus several cycles on establishing the sizing recipe. On a fresh upper, these would be the ones I use to boresight and start up the new barrel.
ETA: This is the time to load at the range with portable hand gear. It saves round trips and you go home with solid answers.
Your initial number is lower than your factory value which sounds suspicious. That may mean the chamber isn't happy with your dies for example.
Will a once fired case go back into the chamber without sizing?
I would measure the shoulder junction diameters and the 200 line diameters to see if your chamber and dies are the reason your number is already shorter than the factory rounds you started with. Those diameters may explain why you had to go lower.
Gargoyle
Finder Outer
I am not saying you are wrong, that's probably the best explanation so far, but I have only needed to fire any brass just once.
RegionRat
Gold $$ Contributor
Sometimes, an AR10 platform barrel is reamed a little too generous for the sake of feeding reliability, and the results are harder on the brass (than necessary).
Other times, the designers spend some effort to squeeze the performance down a little more and as a result, the reamers get closer to the dimensions of a bolt gun and the brass life improves as a result. When we bring a semi-auto platform into our life, we have to accept that they can be much harder on brass when compared to a bolt gun.
It takes some time for any particular semi-auto rifle platform to evolve in terms of refinement of the chamber designs. It doesn't seem so long ago, but I am getting old enough to remember a time before the AR-15/M-16 wasn't as refined as it is now, and The Wooden Guns were on the playing field. In a very short time, The Wooden Guns disappeared, largely due to the efforts of a few good folks who turned the M-16 into a competitive platform for XTC.
The use of a 30-06 or 308 in semi-auto, and how hard/easy those were on brass, took into account the folks who spent some time refining those reamers and dimensions. Given enough time, I am sure they will do the same for the younger cartridges being introduced to the semi-auto platforms.
Other times, the designers spend some effort to squeeze the performance down a little more and as a result, the reamers get closer to the dimensions of a bolt gun and the brass life improves as a result. When we bring a semi-auto platform into our life, we have to accept that they can be much harder on brass when compared to a bolt gun.
It takes some time for any particular semi-auto rifle platform to evolve in terms of refinement of the chamber designs. It doesn't seem so long ago, but I am getting old enough to remember a time before the AR-15/M-16 wasn't as refined as it is now, and The Wooden Guns were on the playing field. In a very short time, The Wooden Guns disappeared, largely due to the efforts of a few good folks who turned the M-16 into a competitive platform for XTC.
The use of a 30-06 or 308 in semi-auto, and how hard/easy those were on brass, took into account the folks who spent some time refining those reamers and dimensions. Given enough time, I am sure they will do the same for the younger cartridges being introduced to the semi-auto platforms.
1.531- 1.532 will be fine, when your ar10 (most likely an DPMS pattern) slams a round in the chamber
It will bump the shoulder back even more.
It will bump the shoulder back even more.
I know. It’s got me scratching my head. Only thing I can thing of is the comment that says new brass takes a couple firings to expand. I just don’t want to get a case stuck in the chamber!!
they seem to chamber. I can’t really tell if they’re going in all the way or not. They seem to be though. I’m going to target 1.533 and try a few out.
Gargoyle
Finder Outer
It is literally a nightmare to get out a stuck live round in the chamber, and out of battery. The only method I've seen successful was a grease gun hooked to a custom threaded adapter. I run small base dies in my AR10 to make sure the entire case down to the head is sized down.
This would be the time to take measurements of new rounds, fired cases, and sized cases. Measure the cartridge base to datum, shoulder, and base at about 0.2". You may or may not need small base dies, depending on the measurements of your cases.
When measuring your cases from cartridge base to datum, the longer ones are more representative of your chamber. Use the measurement of the longer ones to set your full length sizing die.
Excessive headspace can lead to shortened case life, if not case head separation.
jepp2
Gold $$ Contributor
Why would you size 0.003" shorter than factory new brass that functioned just fine in your upper?
How would you know if you never fired until the brass got snug? I'm fairly sure ARs are more problematic in that regard than bolt guns. Just being the Devil's Advocate here.
Bolt guns .002 at the bench .oo4 while hunting Gas guns .004 Minimum and if in the field of fire .007"
Had a friend with a 7mm Browning ( BAR ) RM . We hunted together I built ammo ,but built it to fit My Commercial 3000L Mauser ,which wasn't chambered the same as his BAR . Needless to say I gave him My half of a box of factory ammo ,so we could finish our hunt . I ended up taking a #5 Point Mulie in Utah at measured 465 yd. in 1990 , one shot drop at an uphill quartering with MY custom ammo .
Had a friend with a 7mm Browning ( BAR ) RM . We hunted together I built ammo ,but built it to fit My Commercial 3000L Mauser ,which wasn't chambered the same as his BAR . Needless to say I gave him My half of a box of factory ammo ,so we could finish our hunt . I ended up taking a #5 Point Mulie in Utah at measured 465 yd. in 1990 , one shot drop at an uphill quartering with MY custom ammo .
Coyotefurharvester
Silver $$ Contributor
2nd the suggestion 1.535, set the die off LONGEST brass. Base shouldn't be an issue if all the brass is out off the same chamber. Spent primer method works with AR platforms,too.
Take your upper off, put a piece of fired brass in the chamber, and push the bcg closed by had. If it fits, it ships.
fatelvis
Silver $$ Contributor
Does your rifle have an adjustable gas block? If so, turn it off, so it will not eject at all and fire a few rounds of brass manually ejecting it, and measure them. That will give you a more accurate idea of your chamber's size. This is what I've done in my M1As in the past. Actually, I will do that, neck size only and reload, and fire again using the manual extraction method. Gives an even more accurate chamber fill out. Just make sure your neck sized cartridge is fully seated and bolt completely closed before firing.
NorCalMikie
Gold $$ Contributor
If the "fired brass" re chambers with no issues, you're on the right track. Shoulder bumping is no advisable BUT, if you know what's going on and what to do, it can and is done. 
First time with a slam fire our out of battery round going off and you'll get the message.
And the only difference between a bolt gun and a semi auto is, with a stuck case in the chamber, the bolt gun has a bigger handle to beat on.
And you WILL learn when you draw blood. 
(the voice of experience)
First time with a slam fire our out of battery round going off and you'll get the message.
And the only difference between a bolt gun and a semi auto is, with a stuck case in the chamber, the bolt gun has a bigger handle to beat on.
And you WILL learn when you draw blood. (the voice of experience)
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