How do I know if this handguard will fit?

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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How do I know if this handguard will fit?

Postby engineer40 » Sat Mar 11, 2017 9:07 am

I have 3 AR's now. 2 of the lowers I built myself. But I have not yet changed a handguard myself. What specs do I need to verify to make sure that this handguard will fit my 450 Bushmaster?

http://www.midwayusa.com/product/261262 ... inum-black

My 450 BM is the factory 16 inch version.

Thanks! Any insight is appreciated! :D
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Re: How do I know if this handguard will fit?

Postby plant_one » Sat Mar 11, 2017 10:27 am

you just need to know the size of the gas block.


any proper low profile gas block should fit under that handguard. i've got them on a couple of my AR uppers, they're decent handguards for the $$ and i love the skinny profile
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Re: How do I know if this handguard will fit?

Postby Hoot » Sat Mar 11, 2017 10:29 am

A Mil Spec receiver has a standardized barrel nut thread specification of 1.25-18 TPI. Manufacturers will usually include a barrel nut with their handguard as there are many different and often proprietary handguard to barrel nut thread sizes and pitches, Bushmaster being no exception. My 450 bushmaster barrel nut to handguard thread appears to be 1.875 x 20 TPI.

From the Midway link:
The UTG Pro utilizes a proprietary barrel nut for user friendly installation.


So, if you intention is keeping the the stock Bushmaster barrel nut, then you have the unenviable task of searching far and wide, for a handguard to you liking that also happens to have the 1.875 x 20 thread pattern.

Hoot

It appears I was typing at the same time as plant_one
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Re: How do I know if this handguard will fit?

Postby engineer40 » Sat Mar 11, 2017 2:00 pm

plant_one wrote:you just need to know the size of the gas block.


any proper low profile gas block should fit under that handguard. i've got them on a couple of my AR uppers, they're decent handguards for the $$ and i love the skinny profile



Do you happen to know if the factory gas block on a 16 inch 450 BM upper is considered low profile? Thanks!
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Re: How do I know if this handguard will fit?

Postby Hoot » Sat Mar 11, 2017 2:08 pm

engineer40 wrote:
plant_one wrote:you just need to know the size of the gas block.


any proper low profile gas block should fit under that handguard. i've got them on a couple of my AR uppers, they're decent handguards for the $$ and i love the skinny profile



Do you happen to know if the factory gas block on a 16 inch 450 BM upper is considered low profile? Thanks!


A rule of thumb is, if it fits underneath the handguard, it is considered low profile.

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Re: How do I know if this handguard will fit?

Postby engineer40 » Sat Mar 11, 2017 2:09 pm

Hoot wrote:A Mil Spec receiver has a standardized barrel nut thread specification of 1.25-18 TPI. Manufacturers will usually include a barrel nut with their handguard as there are many different and often proprietary handguard to barrel nut thread sizes and pitches, Bushmaster being no exception. My 450 bushmaster barrel nut to handguard thread appears to be 1.875 x 20 TPI.

From the Midway link:
The UTG Pro utilizes a proprietary barrel nut for user friendly installation.


So, if you intention is keeping the the stock Bushmaster barrel nut, then you have the unenviable task of searching far and wide, for a handguard to you liking that also happens to have the 1.875 x 20 thread pattern.

Hoot

It appears I was typing at the same time as plant_one


Thanks Hoot! I've installed custom barrels on Savage and Remage style bolt action rifles using barrel nuts. On these the barrel nuts have a direct impact on the headspace. If I need to change the barrel nut on a 450 Bushmaster, do you know if it also affects headspace? Sorry I realize this may sound like an ignorant question to those of you better informed on this platform. I've had AR rifles for a while and have done many firearm projects, but have yet to mess with AR uppers at all.

I don't necessarily want to change the barrel nut if I could find a handguard that I like that would fit the barrel nut my 450BM already has. But I'm assuming by your response that there are not many handguards offered that have the 1.875x20 thread pattern?

Thanks!
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Re: How do I know if this handguard will fit?

Postby engineer40 » Sat Mar 11, 2017 2:12 pm

Hoot wrote:
engineer40 wrote:
plant_one wrote:you just need to know the size of the gas block.


any proper low profile gas block should fit under that handguard. i've got them on a couple of my AR uppers, they're decent handguards for the $$ and i love the skinny profile



Do you happen to know if the factory gas block on a 16 inch 450 BM upper is considered low profile? Thanks!


A rule of thumb is, if it fits underneath the handguard, it is considered low profile.

Hoot


That makes sense! The reason I question if the factory gas block is considered low profile is because the factory handguard has the largest diameter I've ever seen. Since it's such a girthy handguard on the rifle now, it seems like a gas block that is not low profile could potentially still fit under it. Thanks!
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Re: How do I know if this handguard will fit?

Postby Hoot » Sat Mar 11, 2017 6:32 pm

I have two SA Model 12's in Laminate Thumbhole stocks and 4 different caliber barrels to swap between them. I get the headspace concern. AR platforms are a lot different than that. As long as you tighten down the barrel nut to the receiver, there is no way the headspace can suffer. It is not set by the barrel screwing in then being set by the barrel nut like a Savage. Its a whole lot simpler than that.

I've seen bigger and smaller diameter handguards than the Bushmaster on AR platforms. Don't worry about buying the handguard you like if it comes with it's own barrel nut. The AR receiver threads are a standard spec. The handguard side of the nut is where there's differences, depending upon obviously diameter, but also, the manufacturer's thread preference. Regardless of the handguard side threads on the barrel nut, the receiver side will fit and will not affect headspace.

If you like that one you referred to on the Midway site, get it. It will work.

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Re: How do I know if this handguard will fit?

Postby plant_one » Sun Mar 12, 2017 9:39 pm

engineer40 wrote:
I don't necessarily want to change the barrel nut if I could find a handguard that I like that would fit the barrel nut my 450BM already has. But I'm assuming by your response that there are not many handguards offered that have the 1.875x20 thread pattern?

Thanks!



just change the barrel nut. AFAIK the bushmaster proprietary barrel nut wont fit much at all - if anything - besides a DPMS or Bushmaster free float handguard. Maybe a hogue too, but dont hold me to that.

the UTG super slim nut is a very nice configuration, even if its not "mil spec" and is proprietary to the handguard. it'll still bolt right up to your upper. and theres nothing to get in the way of the gas tube - the gas tube goes over the barrel nut, not through it. tighten to spec, slide your gas tube in, tighten/align your gas block then just slip the handguard over & tighten the locking screws down.


mil spec should read as "lowest common denominator". that doesnt make them bad, its just a baseline standard. going outside of baseline can pay dividends in a few key areas - barrel, trigger, handguard & stock being the most popular upgrades from mil spec. :)
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Re: How do I know if this handguard will fit?

Postby engineer40 » Mon Mar 13, 2017 6:56 pm

Thanks all! I went ahead and ordered a hand guard. I appreciate the extra confidence that it will fit!
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