Dan,
The M4 feedramps are the barrel's two feedramps continuing into the upper receiver.
A 30 cal bullet does not fit in these two feedramps, so I read that BushMaster (when the company existed) would turn those two feedramps into one larger feedramp. So instead of two skinny feedramps there is one feedramp.
But these feedramps also function as the place where the bolt rotates and locks into place. So, now there is one less place for the bolt to lockup.
I was a bit concerned that this might make the upper unsafe, but if BushMaster did this - it would probably be ok.
In this thread you will see pictures of someone who made these two feedramps a little bit wider looking.
They call it "polishing" but I think they enlarged them.
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Here is a picture of the Bushmaster feedramps.
Looking at his picture, all 8 locking lug things in the barrel are still there.
I did not have this picture when I did mine - and I actually removed the bottom locking/lug system.
So, instead of 8, mine only has 7.
I am probably lucky that the rife doesn't explode
Yes, expanding means on impact.
Hornady sells TAP ammunition to Law Enforcement - many of them using GMX bullets.
HornadyLE.com shows the FBI test results.
The FBI testing procedure was meant for handgun ammunition, but some manufacturers also test their rifle ammunition.
The GMX bullet works great - passes all the tests (sometimes with a little bit too much penetration), retains almost all its mass.
GMX bullets do not expand unless moving over 2000 fps, but this one particular GMX bullet will expand at 1500 fps because it is for the 300 blackout which can't get to 2000 fps no matter how you load/reload it because it just doesn't have enough case capacity.
Howard
My feelings at the time was that the 7.62x39 was a waste - doesn't work - and I would take the risk of ruining it with my dremel and limited knowledge.
I think I fired around 100 rounds testing it - and it worked.
Even after making this nice feed ramp, it still did not work - until I purchased magazines made by C-Products Defense.