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Hey guys! I’m brand new to the forum so hopefully I’m posting this in the correct spot. I’ve been a bushmaster ACR owner for a while now but I recently purchased a 6.8 SPC conversion kit. From what I’ve read 6.8 seems to really stand out in an SBR configuration compared to other cartridges. The conversion kit I ordered is a 16.5” barrel but I was thinking pretty seriously about having it cut down to 12.5” and re-threaded. I don’t have the conversion in hand yet and information online is limited and conflicting. From my research the best I can tell it is chambered in SPC II and the twist rate is 1:11”. Not sure how 1:11 would perform once cut down to 12.5”. Also, being new to the cartridge, I’m wondering what types of 6.8 are safe to use with my rifle and which types are not safe. I have been trying to research as much on my own as I can but it seems there’s a lot of conflicting information out there and very little details on the ACR when chambered in 6.8. Any advice would be appreciated! Thanks guys.
 

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Welcome. You could have posted in the new member introduction area but this is fine. Not sure what conflicting info you are seeing but if it is SPC II then any factory ammo should be good to go. I don't see twist rate causing any problems with the shorter barrel. Do you have an SBR stamp or is it in pistol configuration. That should be the biggest concern about shortening the barrel.
 

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Welcome. You could have posted in the new member introduction area but this is fine. Not sure what conflicting info you are seeing but if it is SPC II then any factory ammo should be good to go. I don't see twist rate causing any problems with the shorter barrel. Do you have an SBR stamp or is it in pistol configuration. That should be the biggest concern about shortening the barrel.
Thanks! I’ll be sure to check out the new member introduction area. The conflicting information was more in regards to the conversion kits actual chambering than anything. Some sites say it’s 6.8 SPC, others say 6.8 SPC II. I would reference Bushmasters website to get an answer straight from the source but unfortunately it no longer exists with everything going on over at Remington Outdoors. My understanding was that it might be unsafe to use certain loads with the original 6.8 SPC but that the SPC II is designed to handle the higher pressures. I’m guessing it will designate on the barrel markings whether it’s a SPC or SPC II? As far as the rifles configuration, it is a stamped SBR. It’s currently running a 10.5” 5.56 barrel. Thanks for the help!
 

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As far as factory ammo most ammo if not all is loaded mild and safe for the original chamber dimension often called spec 1 . The advantages of spec 2 are the longer throat reduces pressure with loads of the same length and allows loading to mag length with most bullets and more powder . I'm not sure what a conversion kit consists of as the barrel , magazine and bolt are all that is needed . The cutting of the barrel would be dependent on how much it cost to do it and what quality the barrel is . An option might be sell the barrel and buy a 12.5" from ARP . Or keep the barrel to use in states not sbr friendly ( if travel might be in the future ) and buy a 12.5" ARP .

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might be less risky n cheaper to buy a 12" rather than having it machined down.
as stated previously, putting a short barrel on a AR lower registered as a rifle, will make it a SBR. consider buying a new lower n register as a pistol.
 

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OK guys. A Bushmaster ACR is not an AR. Nothing interchangeable with an AR except maybe the grip. It is more closely related to an FN SCAR with rotating bolt and Gas piston. It also has quick change barrel capabilities. It was originally designed by Magpul and called the Masada but was dropped and later licensed to Remington/Bushmaster.
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Discussion Starter #7
As far as factory ammo most ammo if not all is loaded mild and safe for the original chamber dimension often called spec 1 . The advantages of spec 2 are the longer throat reduces pressure with loads of the same length and allows loading to mag length with most bullets and more powder . I'm not sure what a conversion kit consists of as the barrel , magazine and bolt are all that is needed . The cutting of the barrel would be dependent on how much it cost to do it and what quality the barrel is . An option might be sell the barrel and buy a 12.5" from ARP . Or keep the barrel to use in states not sbr friendly ( if travel might be in the future ) and buy a 12.5" ARP .

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This info helps a lot as far as understanding my limitations, or lack there of, of the cartridge. Sounds like I’m safe to run any 6.8 SPC. Thanks!
As far as the conversion kit, it consists of a new barrel, bolt, magazine, and charging handle assembly. The older charging handle assembly was only compatible with 5.56 barrels as it was bulky and didn’t allow enough space for larger profile barrels. I’ll give the ARP barrels a look as well. Thanks again!
 

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might be less risky n cheaper to buy a 12" rather than having it machined down.
as stated previously, putting a short barrel on a AR lower registered as a rifle, will make it a SBR. consider buying a new lower n register as a pistol.
I’ll definitely consider purchasing a factory 12”. I was hoping to keep all of the barrels factory bushmaster for collector purposes, but if the quality of the barrel and convenience outweighs the collector value then I may go that route. As far as the legal status, this receiver is already registered as an SBR so no issue there. Thanks!
 

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OK guys. A Bushmaster ACR is not an AR. Nothing interchangeable with an AR except maybe the grip. It is more closely related to an FN SCAR with rotating bolt and Gas piston. It also has quick change barrel capabilities. It was originally designed by Magpul and called the Masada but was dropped and later licensed to Remington/Bushmaster.
View attachment 62869
Woodstock is correct, the weapons are very different. They share a few common parts with the AR but not many. Also, unlike an AR, the upper receiver is what’s serialized on an ACR so that many different lowers can be used with the same short barrels and no need to have a tax stamp for each lower. Just one stamp for the upper. They do however share the same barrel as an AR. They just use different hardware mounted to the barrel. A piston rod, spring, gas block, and gas regulator instead of the traditional gas block and gas tube. Because of this, one beneficial feature is you actually can purchase aftermarket AR barrels and mount the piston system to them for use in an ACR. I guess I should’ve been a little more descriptive when I made the post. With that said you guys have been super helpful and answered pretty much all of my questions. I’m looking forward to joining the 6.8 family and getting familiar with this cartridge. Thanks for all the advice guys!
 

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My earlier post about SPC II and factory ammo was not exactly what I meant to say. If it is SPC saami chamber then almost any factory ammo is fine as well. There were some TAC loads out there that might have been a little hot but as long as you stay away from anything like that you should be fine as well.
So the barrel assy isn't quick change in the ACR? I thought they had caliber change kits that took a few minutes to swap but maybe I was mistaken in that you just swap the entire upper?
 

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My earlier post about SPC II and factory ammo was not exactly what I meant to say. If it is SPC saami chamber then almost any factory ammo is fine as well. There were some TAC loads out there that might have been a little hot but as long as you stay away from anything like that you should be fine as well.
So the barrel assy isn't quick change in the ACR? I thought they had caliber change kits that took a few minutes to swap but maybe I was mistaken in that you just swap the entire upper?
You are correct, the barrel assembly is quick change. I forgot to mention that along with the piston rod, spring, gas block, and regulator, there is also a locking wrench. The barrel itself is not proprietary though so all of those items can be put on any AR barrel for use in an ACR. Below are some images of my setup for reference.
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Sweet setup. How much does it weigh?
 

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Sweet setup. How much does it weigh?
Thanks! I don’t know exact numbers off the top of my head, but it’s HEAVY lol. Way heavier than what a rifle that size should be. I’d say in its 10.5” SBR configuration with polymer Handgaurd, it’s still a solid 1.5-2lbs (guessing) heavier than my go to AR, which isn’t a light AR. I’ll attach a picture of the AR below for comparison. That’s honestly my only complaint about the ACR is it needs to go on a diet. That’s the biggest reason my AR is still my “go to” rifle and my ACR will remain a entertainment/collectors gun. It’s definitely fun to shoot but I wouldn’t want to pack it around in a full size configuration for very long. As an SBR is bearable though. I still use it in carbine matches pretty regularly.

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