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[johnnybravoo77]

Was going to have my handi rifle done in 450bm, but im thinking i would do an ar. Problem is, im not sure of the best path to go about it. Should i buy an oracle then get the barrel and bolt? Or a lower and try to find an upper? Or try to build an upper? Any help or advice is appreciated.

[plant_one]

a standard mil spec AR15 upper wont handle the 450bm. the ejection port needs to be modified to a larger port to allow the brass to eject.

the real trick i see to building one right now is finding an upper with the proper ejection port in it.



i have a buddy who's interested in getting one and he's kind of in the same boat as you at the moment. although like me he's probably going to buy a complete 450bm upper (mine will be here in a few days). He wants something with light recoil that's affordable to practice with that'll be similar function wise to his 450, so im helping him build a 223 from the ground up. then he'll just get his 450 upper and two pins and he's in business.


if you dont already have a 223 ar - you might as well pickup the oracle to start with, and then either build - or buy - a complete 450 upper. Being able to do a full swap with just two pins is really neat. if you haven't built before know that you'll be in to buy some tools to complete a proper assembly of an AR upper. expect to drop $100 or more on tools to help make the assembly go smoothly.

I've got 4 uppers in 3 calibers right now that share the same lower. I dont think i'm going to use the 450 on that lower though as i need to keep the carbine weight buffer in there for subsonic blackout loads. I haven't decided yet if i want to put my A2 stock back on the other lower, or just get a h3(or heavier) buffer for the adjustable stock that's already on it - or just get an adjustable block for the 450 upper and tone it down to mate nicely with the carbine spring/buffer. I might get lucky and the barrel profile will mate up nicely with the red barn armory adjustable gas block i have laying around, but i'm not gonna get my hopes up on that yet.

i think they may have possibly designed one of the worst adjustable gas blocks in history - the locking screw for the adjuster on the side of the block is below the shoulder of the barrel profile at the back of the journal so you have to loosen the gas block and slide it forward to mess with it - but it could be that my initial attempts just paired it with the wrong profile barrel as well. it could be why they dont sell them anymore as well :/

[m113103]

Upper with opened ejection port.
http://www.tromix.com/450-bushmaster-parts.html

[johnnybravoo77]

Upper with opened ejection port.
http://www.tromix.com/450-bushmaster-parts.html

Their site is open in my browser already!

[Al in Mi]

they are not hard to build, you can pick a barrel maker, handguards, gas block. Tromix has most available,

[plant_one]

ya i wasnt trying to suggest it was impossible or anything, just that the list of makers who have a upper milled out for what we need is fairly small

they are not hard to build

i was just making sure the OP was aware that it was an issue to consider when looking to do a 450 build - along with the tool requirements to complete an upper.

i've never assembled a bolt carrier group from the ground up - ive always bought them complete - but otherwise i wholly agree that a new upper build is fairly straightforward.

the trickiest thing i've had to fight so far is the gas tube roll pin installation. that is a royal SOB to do with out a roll pin starter punch and fsb/gas block bench block. thats probably the most frustrating process i've come across when working with uppers, besides driving pins out of my bushmaster milled front sight base so i could switch to a low pro gas block and install a free float handguard. talk about having to swing your purse at it to get those pins moving... sheesh!

[johnnybravoo77]

Do i need an adjustable gas block? Set screw, or clamp on style?

[plant_one]

most system dont use adjustable gas blocks, but they're one of the easier ways to tune a gas system and reduce felt recoil, especailly if you're sharing a lower with multiple calibers.


as i mentioned - having a carbine gassed 300 blackout i need to run the light carbine weight buffer and spring to successfully cycle subsonics. which from what i can tell the carbine buffer is very light for the 450bm. so i'm going to explore using the adjustable gas block i have as a budget option to tune it to that lower.




WRT gas block style - clamp on style gas blocks tend to have the tightest fit ... and therefore the least chance of a leak. Set screw models can be nice for alignment if your barrel is dimpled, but you often need to try several to find one that fits your barrel snug. The set screws are there to prevent movement - both rotation and sliding - more than they are to actually seal the gas system. If you want to use set screw block - some folks will tell you to actually get a block thats too tight and use a butterfly or barrel roll to open them up just so they fit for the tightest seal possible. meaning if you could - sort through the stack of the proper diameter ones for a block that's machined a few thousands under ... ie: a .750 diameter block that is closer to .745-.748 ID.

i like using this style as you can do the barrel roll or butterfly as shown with the same tool head and they're capable of accepting multiple grits of regular sandpaper cut into strips


if you can find the proper grit and diamater, a flap sander works great too

[Avenger10]

Your standard carbine buffer should reliably cycle a properly gassed 450B all day long. No need to be swapping lower parts. Our 450 rifles come mid-gas in 16" and 20" with a .0998 gas port and cycle standard carbine and standard rifle lowers like clockwork when shooting factory pressure loads.

-Bartt

[plant_one]

i'm not concerned about their inability to cycle with a standard carbine buffer and spring. i'm sure its got plenty of gusto to manage that.


my concern was with pairing with a carbine weight buffer like that it could be abusive on the lower (long term) - and possibly my brass (short term) - due to carrier speed and lock time respectively. with the cost of components for this caliber, i want to treat my recyclables as well as i can for maximum life. i would be significantly less concerned about brass life if we were discussing nato 5.56 or 7.62 that can be had for 5-10 cents per (respectively) in bulk lots.

there's a fairly noticeable difference between a standard carbine buffer and a standard rifle buffer - a little over 2oz. in the catalog both of the complete rifles bushmaster offers - the 16 & 20 - come with an A1/A2 style stock on them... which tells me their "standard" buffer weight is the 5.2oz - which would mean a H3 (5.4oz) would be the closest match in a 6 position adjustable system.

ergo - my desire to consider an adjustable block for use on my carbine buffered lower w/ the nice timney & magpul CTR to manage the above issues by limiting the gas vs increasing the reciprocating mass which would interfere with its operation with other calibers. plus the fact that i already have said block sitting in my bin-o-parts already. my other lower has a mil spec trigger and just a MOE stock on it. i also have an A2 stock in the parts bin and put that lower back to a more factory configuration if i decide to go that route, but i prefer the adjustable stocks.