Bolt Carrier Interchangeability

RoBro Cop

New Hide Member
May 4, 2018
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0
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#1
Good evening, gents!

You have all been so helpful, I hope you can tolerate another question from a beginner!

I recently purchased a complete Mk12 Mod 0 upper from Precision Reflex Inc. Unfortunately, the lower it's being mounted to is an early (PreBan) style with the Colt Sear Block similar to the one pictured.
Block.png

The bolt that came with the upper (similar to the far left in the image), is of the newer variety and doesn't have the cut out to clear the block like the old one did ((similar to the far right in the image).
BCG-Shroud-AR15.png

My question is this: Without modifying either the BCG or the sear-block, what would be the best course of action? I believe the bolt was headspaced at the factory so would the best option be to change the bolt between the bolt carrier groups? Would it be better to get a whole new bolt and have it headspaced?

Any help would be appreciated as I'm extremely cautious with safety issues.
 

XLR308

Sergeant of the Hide
Mar 22, 2018
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Grand Junction, Colorado
#3
If the only difference is the cut out in the carrier it doesnt matter what carrier you put the bolt in.
Your question of getting another headspaced bolt is irrelevant since it wont change anything with the carrier.

Personally I would have just bought a new lower so you wouldnt have to deal with the oddball parts.
 
Oct 23, 2009
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Texas
#4
I would sell the lower and get a new one. This isn't a bolt guns. Bolts are interchangeable items. Easiest and best route is to sell the lower and get something that doesn't require random oddball bolt carrier.
 

RoBro Cop

New Hide Member
May 4, 2018
9
0
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#5
What would happen if the sear block accidentally fell out?
Well, nothing. The thing is, the rifle I build off of is a brand-new, unfired PreBan Colt HBAR and its never been fired. In the ban states, its worth about $2500. Should I ever want to sell it, Id like to to continue being in relatively original condition.

If the only difference is the cut out in the carrier it doesnt matter what carrier you put the bolt in.
Your question of getting another headspaced bolt is irrelevant since it wont change anything with the carrier.
That was my thought. I was just looking for confirmation that the bolt is the critical part in headspacing and the carrier is relatively immaterial. Thank you!
 
Jan 15, 2005
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VA
#6
Well, nothing. The thing is, the rifle I build off of is a brand-new, unfired PreBan Colt HBAR and its never been fired. In the ban states, its worth about $2500. Should I ever want to sell it, Id like to to continue being in relatively original condition.
Do you live in a ban state? If not I'd just get a different lower. Lowers are cheap and if you can afford a MK12 upper then some parts to put a good lower together should be a fart in the wind.
 
Likes: W54/XM-388
Jan 6, 2012
3,863
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#7
Somewhat off topic. Did Colt use what would have been an M-16 lower? Looks like the selector labels were irradiated and the block is pinned in the sear holes?
 
Likes: Snuby642
Jan 6, 2012
3,863
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#9
Very much doubt it. Looks like more fuckery from Colt on their older rifles, like large pin lowers. Odd for sure.
 

longshot2000

Sniper's Hide Dealer: CHARLIE'S
Feb 19, 2017
1,230
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Northern VA
charliescustomclones.com
#11
The pre-ban Colt lower had that sear protection bar. You can have it milled-out, and still have a pre-ban lower. If you like the half-moon carrier, then keep it. Use the old bolt or get a new bolt. Lots of options. Like others have said, you can keep the lower or sell it, and buy a new lower. I like the M4 lowers for a Mk12.
 

Tims

Sergeant
Jan 7, 2008
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Orange, California
#12
I have a pre ban Colt lower. It had the sear block like the one pictured. I had the top part of the block milled off and use normal bolt carriers. It was milled flush with the top of the lower receiver.
 

flyer

Sergeant of the Hide
Apr 25, 2018
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#13
If someone is silly enough to pay $2,500 for that, take the money and run away before they change their mind.
 
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