AR 450 barrel lengths and gas

[Bhwoodsman]

I searched through the forum looking for some detail on choosing a barrel length and contour. To me, it seems 16 to 18 inches would be more than adequate for this cartridge. Thoughts? What about the gas length? I read in the forum here and elsewhere about the virtues of carbine and mid gas arrangements and gas port sizes but just don't know enough about this cartridge in the AR. I want it to work but not tear up the brass and the rifle. Any thoughts and specific experiences on this subject.

[Hoot]

Welcome aboard BH!

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I bought a 20" model because all of my hunting rifles have barrels longer than that and I'm used to them. It does provide additional velocity over a shorter barrel which is icing on the cake. Lil Gun, which IMHO is the most appropriate powder for this caliber, doesn't need the extra 4 inches of barrel to finish burning. Both the 16" and 20" Bushmaster uppers come with a carbine length gas system. I can honestly say that for the weight and velocities of bullets I've shot it has not caused any issues of consequence, but I must add that I have a weighted carrier. Regular spring and buffer, but heavier carrier and that is a mitigating circumstance. I would love to try a mid-length gas system, all else being the same, but I do not want to drill a second gas port or buy a custom barrel to get it. If you end up having a barrel made as opposed to buying a Bushmaster upper, if I were in those shoes, I'd go with the mid-length gas system and again a 20" barrel.

Others will no doubt chime in with their experiences and opinions as well.

Hoot

[Bhwoodsman]

Thanks Hoot. It's refreshing to get positive and helpful ideas on a forum. What a concept!

I'm not buying anything from Bushmaster. My concept here is the same as my other AR upper builds. I like to roll my own and stick with basics. Meat and potatoes so to speak.

This fine cartridge in a hunting upper escaped me for a couple of years now due to bolts and suitable barrels. I really liked your weighted carrier project and may have to give it a go on a rather snappy mid-length gas 6.8 carbine as well as a thumper as soon as I can get all the parts gathereed up.

Thanks again.

[wildcatter]

Welcome aboard BH!

You will find two compelling facts about our forum among many positive ones. We are:

Always Helpful
Always Civil

I bought a 20" model because all of my hunting rifles have barrels longer than that and I'm used to them. It does provide additional velocity over a shorter barrel which is icing on the cake. Lil Gun, which IMHO is the most appropriate powder for this caliber, doesn't need the extra 4 inches of barrel to finish burning. Both the 16" and 20" Bushmaster uppers come with a carbine length gas system. I can honestly say that for the weight and velocities of bullets I've shot it has not caused any issues of consequence, but I must add that I have a weighted carrier. Regular spring and buffer, but heavier carrier and that is a mitigating circumstance. I would love to try a mid-length gas system, all else being the same, but I do not want to drill a second gas port or buy a custom barrel to get it. If you end up having a barrel made as opposed to buying a Bushmaster upper, if I were in those shoes, I'd go with the mid-length gas system and again a 20" barrel.

Others will no doubt chime in with their experiences and opinions as well.

Hoot

Hoot,

FYI... I just had several Pressure-Wave events using LG, in another cartridge. It wasn't the 450b and our loads with LG have been exemplary, and is indeed one of the Go-To powders for us. But in this other cartridge, which will be nameless, at this time, LG needed to fill the case and couldn't. So a powder switch was necessary.

Rule of thumb, fill any case as much as possible, and I don't care which powder you're using, a pressure-wave-event can occur (Detonation, look it up) go for 100% case fill or close and then don't over pressure the cartridge when you do this..

..t

[MarkCO]

I'm also a tinkerer, as Wildcatter can attest to, and took an alterantive path to my .450BM build after testing a lot of the suggestions herein. I wanted 16" and felt carbine-length gas gave me more room to tinker with. I went with a mil-spec carrier, standard buffer, +25% buffer spring and adjustable gas. I have the gas volume turned way down and get 100% function. I felt that reducing the gas volume was a better solution than adding mass to the reciprocating assemblies. It will reduce wear long term while taming the recoil and keeping the bolt locked for a sufficient time.

When I build another, I will go 18" and mid-length gas, looks to be about the best trade-off across the board. If I went 20", I would be really tempted to try rifle length gas. However, it might not be reliable and you might end up with more recoil, and certainly a jumpy recoil impulse unless you went with a lightened carrier.

BTW, I have only used Lil'Gun for my loads. I like it and already had it from working with my .414 SuperMag...see no reason to try anything else at this point.

[Hoot]

I'm also a tinkerer, as Wildcatter can attest to, and took an alterantive path to my .450BM build after testing a lot of the suggestions herein. I wanted 16" and felt carbine-length gas gave me more room to tinker with. I went with a mil-spec carrier, standard buffer, +25% buffer spring and adjustable gas. I have the gas volume turned way down and get 100% function. I felt that reducing the gas volume was a better solution than adding mass to the reciprocating assemblies. It will reduce wear long term while taming the recoil and keeping the bolt locked for a sufficient time.

When I build another, I will go 18" and mid-length gas, looks to be about the best trade-off across the board. If I went 20", I would be really tempted to try rifle length gas. However, it might not be reliable and you might end up with more recoil, and certainly a jumpy recoil impulse unless you went with a lightened carrier.

BTW, I have only used Lil'Gun for my loads. I like it and already had it from working with my .414 SuperMag...see no reason to try anything else at this point.

It's funny that Lil Gun, 110, 296, 4227 and 1680 are not just good choices for fat calibers and straight wall cases. The 7.62x40WT also prefers those powders.

It's too bad the industry machine chose to throw their weight to the 300 AAC instead of the 7.62x40WT. It has more case capacity for the same parent cartridge. There's an argument to being quick to the water trough.

Hoot

[Bhwoodsman]

Like MarkCO, I'm a bit of tinkerer as well. I too like the idea of an 18" mid-gas barrel as a starting point. But I don't know anything about how these thumper catridges will work this AR action. So if I can get a 16" mid-gas to work well, I would consider that too if I'm going to get good reliable operation for the long haul. Sight radius is important too as I will probably use metallic sights on this heavy cover rifle/carbine. I don't use muzzle devices on woods hunting barrels. More weight and length. Adjustable gas blocks may be needed to tailor specific loads to the chosen gas length and port size. I use them on 6.8 x 43 and a wildcat 6mm on the same case. Remarkable control with heavy charges and heavy high BC bullets. But, once set for a sweet load, the adjustment feature is rarely used.

I use M16 carriers in my upper builds. One of my lowers used primarily with the 6.8, employs a heavier Wolf buffer spring. Looks like this lower may be my go-to for the 450. I also love Lil Gun. It looked like a very good powder for the 450. But for me, this powder is the best I can use in my .22 Hornet. Can't get double charges in the case. Accuracy is greatly improved. So looks like 8 pounders.

Thanks all for the discussion.

[45r]

I ripped the rims off a couple handloads using loads that are OK with weighted BC's.I reduced the LG load a couple grains and everything GTG.Accuracy fine and recoil is easy to handle to me.Don't see any need for a MB.I use a lower off a 24 inch colt h-bar elite.I think my 20 inch upper is way overgassed.If I build a 450 I'd use a 20 inch barrel with rifle gas or mid-length with adjustable gas block.
I'm going to stick with light loads since I don't want to use a heavy BCG.Plenty of power with the 450 and the light loads are easier on the rifle.If I was to buy a stock 450 and didn't handload I'd get a 16 inch since the 20 seems overgassed.