Why is a full auto bolt carrier sought after in a semiauto target rifle?

We shoot 600 to 1000 yards with 80 and 90gr bullets pushing the envelope on pressures to get as much velocity as possible.  The heavier carrier delays the action reducing the pressure and prevents blown primers.  David Tubb also sells weights you can add to your semi-carrier to increase the weight.

www.davidtubb.com/ar15_cws.html

This link to his product also does a better job of explaining what I just said.

Good question and welcome to the site!

IIRC, it is because the FA carrier is of the correct mass for the design.  Semi carriers do not have as much mass as FA carriers.  I don't know of anyone who has had problems due to using a SA carrier, but some people do run them for added security.  It won't hurt anything to run a FA carrier, but I wouldn't run out and replace all my SA carriers.  I believe this is why, but please correct me if I am wrong.  

Besides the issue of the mass of the bolt,
the shaved bolt tries to move down and back rather than straight back,
so it causes uneven wear.
If that makes sense?

in reality?

Because it is cool or "because I can".

That's it.

I run FA Carriers in all my rigs. That being said, there is only .3 oz difference between a SA Stag and an LMT FA Carrier. I weighed both and the Stag is 9.0 oz and the LMT 9.3 oz. This is the carrier alone; without the bolt, firing pin etc.

Not to hijack the thread, but can anyone (like ME) run a full-auto bolt carrier if all trigger parts are semi-auto parts and the lower is not altered in any way for a sear?

yes it is legal to run a full auto carrier in a semi auto ar15.

Yes you can...

Funny, the way I read that letter, especially the "particular paragraph of item #3" they reference, it sounds a lot more like they are saying "don't do this" to me.  If they say "don't use M-16 bolt carriers in the assembly of AR-15 rifles", I would assume they mean as such.

They also state that they cannot "definitively tell you" that an M-16 carrier will make an AR-15 fire full auto.  Well, in the standard sense, it will not.  However, if the disconnector is removed from said AR-15 (ATF: "readily restorable" to full auto), the ramped M-16 bolt carrier could OCCASIONALLY allow hammer follow-through induced "doubling" of certain rounds (ATF: full auto = "more than one shot per trigger pull").  So the ATF Firearms Expert takes your AR-15 to which YOU added "M-16 parts" and he removes the disconnector (which they can justifiably do) and after several attempts gets it to fire a 2-shot "burst" with soft-primered sporting ammo.  Guess what?  You are now going to see them in court on an illgal M.G. charge, because the rifle "fired full auto" due to the M-16 part you added.  It is not reliable automatic fire and is totally unpredictable and dangerous even, but it meets the definition of "full auto" if it happens even one time during ATF testing of that "readily restorable to full auto" rifle.  

It is best to stick with notched hammers and unshrouded bolt carriers (ala SP1 pattern AR-15 parts) for this very reason.  Plus, you can get the same effect with a bolt carrier weight in a semi carrier, if you feel the need.

So, why is Colt equipping their current carbines with M-16 bolt carrier groups? It would seem to me that since they have been relatively shy of crossing the BATF/Gov't. firearms issues in the past that they certainly would not risk equipping civilian accessible rifles with such a potentially evil part. I called our local area BATF office and asked about the legality of the M-16 carrier in an AR-15 and was told verbally that it was not illegal to do so.

Quoted: Funny, the way I read that letter, especially the "particular paragraph of item #3" they reference, it sounds a lot more like they are saying "don't do this" to me.  If they say "don't use M-16 bolt carriers in the assembly of AR-15 rifles", I would assume they mean as such. They also state that they cannot "definitively tell you" that an M-16 carrier will make an AR-15 fire full auto.  Well, in the standard sense, it will not.  However, if the disconnector is removed from said AR-15 (ATF: "readily restorable" to full auto), the ramped M-16 bolt carrier could OCCASIONALLY allow hammer follow-through induced "doubling" of certain rounds (ATF: full auto = "more than one shot per trigger pull").  So the ATF Firearms Expert takes your AR-15 to which YOU added "M-16 parts" and he removes the disconnector (which they can justifiably do) and after several attempts gets it to fire a 2-shot "burst" with soft-primered sporting ammo.  Guess what?  You are now going to see them in court on an illgal M.G. charge, because the rifle "fired full auto" due to the M-16 part you added.  It is not reliable automatic fire and is totally unpredictable and dangerous even, but it meets the definition of "full auto" if it happens even one time during ATF testing of that "readily restorable to full auto" rifle.   It is best to stick with notched hammers and unshrouded bolt carriers (ala SP1 pattern AR-15 parts) for this very reason.  Plus, you can get the same effect with a bolt carrier weight in a semi carrier, if you feel the need.

also, the ramp is no longer a big thing, lots of AR15 Bcs have the pin shrouded, so a slam fire is possible with those as well

Quoted: Why is a full auto bolt carrier sought after in a semiauto target rifle?

So I can slap my upper on an M16 lower and shoot full auto.

Quoted: So, why is Colt equipping their current carbines with M-16 bolt carrier groups? It would seem to me that since they have been relatively shy of crossing the BATF/Gov't. firearms issues in the past that they certainly would not risk equipping civilian accessible rifles with such a potentially evil part.

I believe Colt started using the M-16 carriers in newer AR carbines due to problems with the firing pin retainers either breaking or bending in carbines equipped with unshrouded (AR) carriers.  I'm not sure why this never was an issue with earlier rifles/carbines, but Colt saw fit to solve the problem with the carrier swap.  The fact that Colt asked the ATF this question and got a run-around answer makes me a bit nervous to follow in their footsteps.

It is true that many factory assembled AR types would be capable of slam fire.  It is also true that it would be extremely unlikely that you would be persecuted due to this issue alone.  I do recommend awareness and discretion when dealing with Big Brother though, especially over something which can cause far more trouble than benefit.  

The last Colt 6920 I bought, lovingly named Grimace because of its purplue hue, came with a "sporter" bolt. Others I have came with the M16 bolt...the gun was a factory new 6920HB. Anyone know why this particular gun, despite being brand new, has a semi carrier? I found it strange myself...I honestly could care less and wouldn't do a thing about it, but I am curious.
Anyone know for sure whats up with the B/C situation in my Colt and others and why thier is a discrepancy? Is this the work of Colt or distributors?