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VA, USA
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Posted: 4/5/2024 2:26:46 PM EDT
[Last Edit: ahcollier1991]
I have an old Bushmaster XM-15 that I dragged out of the gun locker and want to fiddle with.
First order of business is to get rid of the Mil-Spec Front Sight Tower & Gas Block in favor of a low profile gas block. As I shop for gas blocks, I note that they come in a variety of diameters. Two questions: 1) What’s the external diameter of my Mil-Spec barrel? Is it 0.75 inches? 2) Where can I look that up since BM is defunct? Thanks, Andy |
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Posted: 4/5/2024 2:47:05 PM EDT
[Last Edit: WUPHF]
[#1]
A simple cheap set of calipers would tell you for sure.
If you don’t have calipers (and don’t want to go spend $10-$20 at your local HF), it’s typically pretty easy to tell. .625 journals basically only come on LW or pencil profile barrels. Basically everything else (that isn’t a bull barrel or precision barrel) is going to be .750. Hell, you can usually just tell by looking at it. As for looking it up, I’m not really sure. A picture would help. |
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Posted: 4/5/2024 2:49:05 PM EDT
[Last Edit: bfoosh06]
[#2]
If you have a 3/4" combo wrench, that will fit on a .750 journal.
As for removing the front sight... remember those pins are typically tapered and can only be driven out from one direction. Soak the pins now, with Kroil... and you might need to use a larger than needed punch to "start" the pins moving.... then swap to a smaller diameter punch. |
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CA, USA
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Posted: 4/5/2024 6:38:45 PM EDT
[#3]
Originally Posted By ahcollier1991: First order of business is to get rid of the Mil-Spec Front Sight Tower & Gas Block in favor of a low profile gas block. As I shop for gas blocks, I note that they come in a variety of diameters. Two questions: 1) What’s the external diameter of my Mil-Spec barrel? Is it 0.75 inches? 2) Where can I look that up since BM is defunct? I can't help you with dimensions, but if you can't find a good match you can chop the front sight down with an angle grinder and a Dremmel; https://youtu.be/5sBqDpi-Cb4?si=fPUlMq1OMMJX8-or |
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Posted: 4/5/2024 6:54:35 PM EDT
[#4]
Find the rail you want, buy it & send the rail with the upper to John ThomasHave him cut the existing one down & mount your rail. Nothing will fit better/be more secure than a pinned gas block.
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Posted: 4/25/2024 3:38:39 PM EDT
[#5]
Just so you know, Bushmaster has been back up and running for a few years now.
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Posted: 4/25/2024 10:14:51 PM EDT
[#6]
Quote HistoryOriginally Posted By VLODPG: Find the rail you want, buy it & send the rail with the upper to John ThomasHave him cut the existing one down & mount your rail. Nothing will fit better/be more secure than a pinned gas block. This is the way. |
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TX, USA
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Posted: Today 6:49:46 AM EDT
[#7]
Quote HistoryOriginally Posted By VLODPG: Find the rail you want, buy it & send the rail with the upper to John ThomasHave him cut the existing one down & mount your rail. Nothing will fit better/be more secure than a pinned gas block. This is a great option because your barrel was not parkerized under the FSB and you will need to cover the pin holes and areas of bare steel. A normal low profile gas block won't work. Also, the FSB requires quite a bit of reprofiling to fit under a rail. You can't just chop the A-frame, bayonet lug, and sling swivel... you must contour the base where the pins are. This makes a refinish mandatory because not much of the original finish will be left if the FSB chop is done right. |
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Posted: Today 8:08:54 AM EDT
[Last Edit: TGWLDR]
[#8]
Quote HistoryOriginally Posted By DevL: This is a great option because your barrel was not parkerized under the FSB and you will need to cover the pin holes and areas of bare steel. A normal low profile gas block won't work. Also, the FSB requires quite a bit of reprofiling to fit under a rail. You can't just chop the A-frame, bayonet lug, and sling swivel... you must contour the base where the pins are. This makes a refinish mandatory because not much of the original finish will be left if the FSB chop is done right. THIS is a great option for converting previous FSBGB barrels to LPGB. It covers the taper pin channels entirely. Hit the exposed metal with some cold blue if so inclined. Easy enough to pin if desired as well.  Attached File |
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