I have before me two different bolt-carriers.

#1. This one came with my DPMS Panther Bull Deluxe. The carrier nearly completely encloses the firing pin as I thought was standard.

#2. This carrier came with a BM M4 upper recently. It appears different in that the bottom of the carrier exposes the firing-pin right up to where it seats into the bolt. Compared to the DPMS, the BM has less 'meat', having a larger cut-out, and ramping up to the bolt.

If this makes any sense to the reader, I was wondering....

Why or what's the difference?

From an engineering standpoint the lighter bolt in the M4 would be because of the shorter time for gas pressure to bang the bolt back. A lighter bolt requires less gas to operate.

MDOhmart,

""#1. This one came with my DPMS Panther Bull Deluxe. The carrier nearly completely encloses the firing pin as I thought was standard.""

this one sounds to me lyke it might be for a select fire weapon.., burst/auto fire ???????

There are three different types of semi-auto bolt carrier.  Go to biggerhammer.net to see the difference between the standard semi-auto and the select-fire bolt carrier.

#1)  The standard AR-15 bolt carrier.  This is the "mil-spec" carrier used by Bushmaster and most other manufacturers.  The back end of the carrier has a small area that is circular.  There is a ramp or relief cut on the undersurface leading to the firing pin.  Found in early Colts and most others.

#2)  Colt HBAR bolt carrier.  The back of the carrier is a semi-circle, no complete circle.  No firing pin relief cut.  I don't know why Colt did this?  I guess it made the gun less "evil" like the sear block & all the other Colt BS.

#3)  Colt Sporter bolt carrier.  Found in all new Colts.  The back of the carrier is semi-circular.  There is a firing pin relief cut.  Again, a abberation unique to Colt.

Bill