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Posted: 12/28/2025 1:05:29 PM EDT
| I want to build a lower for my new BM .450 Upper. I know I will need a full length buffer tube for the A2 Stock. What do I do for the buffer? Do I need to use a rifle length buffer or will a carbine buffer work? |
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You need a rifle length buffer tube and a rifle length spring, and a standard rifle buffer should work. You see differences in length between A1 and A2 stocks... this has nothing to do with the buffer tube. The extra length is handled with a drop in spacer. Here's my 450. I'm pretty sure it's an A1 stock, but I honestly lose track.  " />
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Originally Posted By ELittlefield: You need a rifle length buffer tube and a rifle length spring, and a standard rifle buffer should work. https://i.imgur.com/o7kO5H5.jpeg" target="_blank">https://i.imgur.com/o7kO5H5.jpeg ^ | This for the simple, straightforward answer. I'm gonna ramble a bit more for OP or anyone else reading this, though. To complicate things, it is also possible to put a spacer in the back of the rifle buffer tube to reduce its interior length to 7" (carbine length) or 7.75" (A5 length), then run a buffer & spring setup appropriate for the shortened length. I'd advise against that, as the longer systems (rifle and A5) have what is termed a wider operational envelope. (Essentially that means there's more space for more weight to reciprocate and for longer springs with more coils to function without binding.) But, if you want to get even more complicated for tuning later on, there the caveat that not all rifle length AR-15 buffer springs are created equal (nor are all carbine springs). E.G., Griffin Armament's M4 +15% carbine spring specs out closer to a rifle spring than a carbine buffer spring. Your shooting experience with, for example, a Wolff +15% XP Rifle buffer spring at 7.75 lbs. closed/13.25 lbs. open will have a very different feel (probably snappier) from a KAK Flatwire clocking it at 6 lbs. closed/11 lbs. open. I ran across this post: https://forum.308ar.com/topic/34993-compiled-buffer-spring-data/ by a fella named Dewey Mack when looking for some .308 spring info, and ran across this chart provided by him: AR buffer springs - AR15 springs. Some springs are stronger, some are weaker, some have larger variances between open and closed bolt force, and some have smaller such variances. And for anyone else stumbling upon this that may not know, while the amount of gas is a factor in determining buffer weight for any AR, the length of the buffer system is not tied to the length of the gas system. For anyone reading this while building: For a simple compatibility check on any AR's buffer length, install your buffer tube to the lower, then install the buffer + carrier tail into the buffer tube w/o spring (just drop in the buffer and slide the carrier tail in until they bottom out), and ensure there's a small gap between the gas key and the buffer tower of your lower. If there's a long gap (>~1/4") you've likely got too long of a buffer. If the carrier gas key contacts the buffer tower, your buffer is too short for your buffer tube. If the carrier + buffer bottom out further forward once the buffer spring is installed, it means the spring is binding and it is too long for the system and will be damaged with use. Edit: had buffer tube lengths off by 1/4". fixed |
| Thanks for all the advice. I will start off with a vanilla rifle stock/buffer tube length and go from there. I was amazed that one of my lowers worked the first time out. I seriously expected a single shot rifle for my 1st experience with .450 Bushmaster. I will get the stock kit and report back if it works. What I have now works, however, as I am going into my sixth decade of life, recoil mitigation is becoming more of a priority than it used to be. That is why I am building a lower with a full length rifle stock. |
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Good luck! Also remember, a standard rifle buffer comes with steel weights in it, but you can increase the buffer weight by disassembling it and swapping in some tungsten weights, as an alternative to or in combination with increasing spring strength. Some places, like B Kings, sell rifle buffers preconfigured with heavier weights. The cycling system is a triangle built out of gas, spring, and mass. Balance your triangle and be at one with the cycle. If you're going for the least recoil possible and you're okay with a little more weight at the back of the gun, I'd start as you said you are: vanilla rifle buffer, spring, & tube. Then I'd increase the buffer's weight as much as you can. Changing spring doesn't increase the overall weight of the gun much, but adding a few ounces to the buffer does. This will reduce the overall recoil, and it will slow the cycle and spread the recoil from cycling out over a longer period. None of that is to discount Couch-Commando's advice -- use it in conjunction with that. |
| I've ordered the PSA rifle OD stock kit. I wish that lower cost wooden stock guy was steel making them as this would be the appropriate AR15 to do this with for looks. One thing for sure with this big bore is that either your shoulder or wallet will begin to scream if you fire too many rounds. In a way I am thankful that the largest working magazine for .450 BM only holds ten rounds. I've still not decided on a name for Jeff Cooper's "Thumper" label. "Bear Spray" or maybe "T-Rex Ender" are in my mind right now. |
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Originally Posted By txsgar15a2: I've ordered the PSA rifle OD stock kit. I wish that lower cost wooden stock guy was steel making them as this would be the appropriate AR15 to do this with for looks. One thing for sure with this big bore is that either your shoulder or wallet will begin to scream if you fire too many rounds. In a way I am thankful that the largest working magazine for .450 BM only holds ten rounds. I've still not decided on a name for Jeff Cooper's "Thumper" label. "Bear Spray" or maybe "T-Rex Ender" are in my mind right now. I keep contemplating a wood stock for my 450 as well. It just seems right. |
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