FSB swap (should I buy a lottery ticket?) and questions

[mechelaar]

So I purchased a Colt SBR barrel without FSB for a good price from the EE. I was planning on putting a low profile gas block on it and calling it a day, but I noticed the date code was a month off from a 6920 barrel I had. I figured, what the heck, maybe they used the same jig. I put the 6920 FSB on the SBR barrel and surprisingly, it was about a 99% fit. Looking down the pin holes, I could see it was slightly off (probably by less than 1/128 inch). It was close enough that I was able to push the pins through by hand. Since I was this far, I gave it a couple light whacks with a hammer and they went all the way through. They seemed slightly tighter than normal to go through, but I certainly didn't hear screaming and twisting steel or anything. The FSB sits tightly and seems to be perfectly in position. I haven't put any rounds through it to ensure proper gas flow, but lets assume everything works as it should.

Here are my questions:
I know a shaved FSB > lo pro gas block. But is a slightly mismatched FSB > gas block?
Does this whole thing seem kinda bubba'd to you guys? I would like this to be a nice (reliable) rifle.
I have always heard that FSB swaps are statistically nearly impossible. Do I need to buy a lottery ticket?

Thanks for the help, all.

edit: typo

[discreet]

So I purchased a Colt SBR barrel without FSB for a good price from the EE. I was planning on putting a low profile gas block on it and calling it a day, but I noticed the date code was a month off from a 6920 barrel I had. I figured, what the heck, maybe they used the same jig. I put the 6920 FSB on the SBR barrel and surprisingly, it was about a 99% fit. Looking down the pin holes, I could see it was slightly off (probably by less than 1/128 inch). It was close enough that I was able to push the pins through by hand. Since I was this far, I gave it a couple light whacks with a hammer and they went all the way through. They seemed slightly tighter than normal to go through, but I certainly didn't hear screaming and twisting steel or anything. The FSB sits tightly and seems to be perfectly in position. I haven't put any rounds through it to ensure proper gas flow, but lets assume everything works as it should.

Here are my questions:
I know a shaved FSB > lo pro gas block. But is a slightly mismatched FSB > gas block?
Does this whole thing seem kinda bubba'd to you guys? I would like this to be a nice (reliable) rifle.
I have always heard that FSB swaps are statistically nearly impossible. Do I need to buy a lottery ticket?

Thanks for the help, all.

edit: typo

As long as it's straight, the gas tube hold lined up, and the pins were tight... what else are you worrying abou? The reason they say FSB's are greater than low pro's is because they are PINNED. That is the only reason, other than having a fixed sight. If your running a rail etc and shave the fsb, there is literally nothing more special than a pinned low pro.

[mechelaar]

As long as it's straight, the gas tube hold lined up, and the pins were tight... what else are you worrying abou? The reason they say FSB's are greater than low pro's is because they are PINNED. That is the only reason, other than having a fixed sight. If your running a rail etc and shave the fsb, there is literally nothing more special than a pinned low pro.

Thanks for the response. I am planning on shaving the FSB. I guess I'm wondering if there is anything about the slight mismatch in the holes that could cause long term reliability issues (pins working loose, etc.). I'm fairly confident it will run as is, but I would like the gun to be as reliable as possible. I have no problem paying to pin a gas block. If a pinned lo pro gas block is going to be more reliable than the current set up, I'll go with the gas block. But if the consensus is that this FSB is just as good, I'll keep it as is.

[samuse]

Shave your FSB and you have a dual pinned low-pro gas block. If you were able to get the pins in by hand and reasonably set 'em with a hammer, I don't think they're coming out. Probably really not coming out!

Here's a FSB I shaved Tuesday. I cut the sight post off with a Metabo, roughed it in with a 4 1/2" angle grinder, finished it off with a 8" round bastard and 10" half round bastard file and some emory cloth. It will fit under most any rail.

http://i851.photobucket.com/albums/ab76/sammuse/para%20venta/004_zps6ded6b7d.jpg

http://i851.photobucket.com/albums/ab76/sammuse/para%20venta/002_zpsb930b8cb.jpg

http://i851.photobucket.com/albums/ab76/sammuse/para%20venta/003_zpse82275db.jpg

[sugerwater]

Your final answer will come from at the range, otherwise your fsb pins probably formed to the out of position holes. May be harder to drive out the next time and probably not reusable again. If all is tight and square your GTG.

[markm]

The intake on an FSB is much larger than the actual gas port. I can't imagine there being any gas flow issues with this project. Especially since it's Colt to Colt.

[SpankMonkey]

Buy a lotto ticket.

If it shots and works you are golden.

[mechelaar]

Thanks for the replies, all. I popped the pins back out. They were a bit harder than usual to remove (still not as big of a PITA as BCM pins). Looking through the pin holes now, there is no longer any protruding metal (I assume the pins pushed it back), just a little gap at the 12:00 position where the barrel sits a tiny bit higher than the FSB. I'm gonna put it back together, get it to the range tomorrow, and see how it works. If everything is good to go, I'll probably shave it down and call it a day. If anyone is interested in pics, let me know. Otherwise, thanks for the help.

[Col_Crocs]

I think you'll be fine. One thing that's been bugging me though... If the FSB is from your 6920, what do you have on it now? Low pro?

[mechelaar]

Just to update, I took the rifle to the range. No issues.


I think you'll be fine. One thing that's been bugging me though... If the FSB is from your 6920, what do you have on it now? Low pro?

Good catch. There's nothing on it. I bought the rifle with the intent of SBRing it and replacing or cutting the 6920 barrel. I'm probably going to sell the stripped 6920 barrel on the EE.