Quote History Quoted:
May or may not matter...
In my experience, IMI
ammo is loaded to full power/velocity Mil Spec. It is
5.56 and is designed to be fired in a mil spec
5.56 chamber. I burned through a lot of it some years back and it was good ammo but was not tollerant of any variation in chamber or gas system. It ran great in my Colt factory 14.4 inch gun. It was giving pressure signs in my home built Recce rifle with a CLE chamber.
Keep in mind...unless you have gauged it, it may or may not actually be the chamber that is marked on the barrel.
YMMV, just food for thought.
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+1. Per "The
AR-15, Volume 4", pg. 105, peak chamber pressure of 556 ammunition in 223 chambers can be measured as high as 75k PSI and I would think could cause the problems noted in this post. Of course hot ammunition- either "loaded hot" or with a high internal temperature may cause the same thing. In my experience and anecdotal evidence, many barrels marked as 556 actually measure to be a solid 223 chamber.
My barrels (measured using Ned Christiansen's .223/5.56? Gage):
- BCM 16" BFH, chrome lined, 556, 1:7= measures a tight 556. Fires IMI (my chosen primary supplier) including 00225/G fine and is the only barrel I have fired this in.
- Bushmaster 16" pencil circa 2003, 556, 1:9= measures 223
- PSA 10.5", 556, 1:7= measures 223
- Green Mountain 10.5", chrome lined, 556, 1:7= measures 223 ( I will fire the IMI 77 OTM in this barrel in the near future)
My guess is the less expensive barrel manufacturers change the reamers less often than required to maintain 556 chambers with the 556's longer freebore/throat, which is also where my 556 barrels that measure as 223, touch the gage.
I will chronograph loads in the BCM BFH and Green Mountain barrels this spring after it warms up and I have completed the new rifle range I am building.