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Old 07-07-2020, 08:20 AM   #1
pontiac
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Default Swapping AR Upper Receivers

Wanting to get opinions on having one lower and swapping out upper assemblies to essentially have a 2-in-1 gun.

From what I understand, when going from 556 to 6.8SPC I will need a different BCG, barrel and magazine. Is there anything else I would need that is unique to the 6.8? Any need to change or adjust the buffer system?

I am thinking for convenience it would be nice to have 2 assembled uppers instead of using 1 receiver and swapping barrels out. Other than cost is there any down side to this approach?

BTW if anyone has a complete upper in either 556 or 6.8 with 16" barrel, I am looking for one.

Thanks.
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Old 07-07-2020, 08:39 AM   #2
Ryan C
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I have a set up just like you mention. No need to change the buffer system between calibers. If the 6.8 is more bang than you like, you can put in a heavier spring and use it on both uppers. I think all you actually need to change is the bolt but its quicker/easier to have the BCG altogether for each caliber.
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Old 07-07-2020, 09:01 AM   #3
Lsube0555
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Complete uppers are cheaper enough to do this. I have a 6.8 and a 556 upper. I don’t really use the 556. Unless just plinking at range or for a shtf situation. Optics stay on upper so all I gotta do is switch and go.
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Old 07-07-2020, 07:05 PM   #4
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Thanks for the input.
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Old 07-08-2020, 06:29 AM   #5
trophy8
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Quote:
Originally Posted by Ryan C View Post
I have a set up just like you mention. No need to change the buffer system between calibers. If the 6.8 is more bang than you like, you can put in a heavier spring and use it on both uppers. I think all you actually need to change is the bolt but its quicker/easier to have the BCG altogether for each caliber.
The buffer weight has more to do with the reliable operation of the rifle than to control recoil. You’re controlling bolt speed. And there are many factors in that. Gas port size. Suppressed vs not. Barrel length and gas system length. Lots goes into that. Just putting a heavier buffer in one to lower recoil isn’t that way to go about it.

I run heavier buffers in some rifles depending on my above mentioned factors. Grendel and 6.8 are different than a 223/556.

OP yes you’ll need a different bolt. The carrier is the same. Mags on the 6.5 and 6.8 are different than a 223/556 as well.

I’d lean towards a separate rifle altogether. You can make different uppers work. But for a few hundred difference you don’t have to deal with it.
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Old 07-08-2020, 07:27 AM   #6
wsteffen
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I do this with a 556 and a 300blk upper when I travel. I actually have two lower assemblies but one of them has been made NY compliant for when we travel.
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Old 07-08-2020, 09:02 AM   #7
RattlesnakeDan
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Just pick up a 6.8 upper complete from Palmetto State or similar and buy a 6.8 mag and you are good to go. Take both uppers with you when you go somewhere so you have a back up just in case or the critters you hunt changes mid stride.
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Old 07-08-2020, 11:48 AM   #8
ahlongslide
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I switched multiple uppers on a SBR lower for a month or two before giving in and buying separate pistol lowers. They're so cheap that there isn't much of a benefit in going the switching route.
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Old 07-08-2020, 04:19 PM   #9
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Quote:
Originally Posted by ahlongslide View Post
I switched multiple uppers on a SBR lower for a month or two before giving in and buying separate pistol lowers. They're so cheap that there isn't much of a benefit in going the switching route.
This was what I did for a while when I first got into AR's and ended up going the same route and just getting lowers for everything since they're so cheap and have I also no idea when the crown will stop allowing us to buy/own them.
Plus once you get enough of them it makes it impossible for the wife to tell if its a new one or not LOL!
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Old 07-08-2020, 05:51 PM   #10
mrc
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I did this for a little bit and gave up and shoot everything with my 5.56 and 77 grain ammo. I just got tired of multiple components and ammo.
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Old 07-08-2020, 08:25 PM   #11
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Well I succumbed to the lure of the Grendel. Presently have an upper on the way, along with 3 E-Lander magazines, 500 Rounds of Wolf steel case and a box of Hornadys on the way.

I will Start with this and work on a 556 later.
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Old 07-10-2020, 04:18 AM   #12
sir shovelhands
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Quote:
Originally Posted by ahlongslide View Post
I switched multiple uppers on a SBR lower for a month or two before giving in and buying separate pistol lowers. They're so cheap that there isn't much of a benefit in going the switching route.
I run 5 uppers on one SBR lower: that seems like an expensive proposition if you don't use stock triggers.

What price are you getting for lowers with a full lpk and pistol buffer?

Last edited by sir shovelhands; 07-10-2020 at 04:20 AM.
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Old 07-10-2020, 06:21 AM   #13
Tony Pic
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Quote:
Originally Posted by pontiac View Post
Well I succumbed to the lure of the Grendel. Presently have an upper on the way, along with 3 E-Lander magazines, 500 Rounds of Wolf steel case and a box of Hornadys on the way.

I will Start with this and work on a 556 later.
Good luck and have fun. Run the gun wettish/oiled initially shooting the steel case ammo for break in.
I do not have a Grendel, but in the calibers, I do shoot steel case in. 223 and 762x39, I have broken case extractors because I have had shoulders ripped off steel case ammo and the case left inside the chamber rendering the gun useless until the broken case is extracted. I still shoot steel, just keep the broken case extractors on hand when I do.
https://www.brownells.com/gunsmith-t...311-49638.aspx
I agree with the other guys. Get multiple lowers and welcome to Lego, For men

Last edited by Tony Pic; 07-10-2020 at 06:24 AM.
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Old 07-10-2020, 06:40 AM   #14
Radar
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Quote:
Originally Posted by trophy8 View Post
The buffer weight has more to do with the reliable operation of the rifle than to control recoil. Youíre controlling bolt speed. And there are many factors in that. Gas port size. Suppressed vs not. Barrel length and gas system length. Lots goes into that. Just putting a heavier buffer in one to lower recoil isnít that way to go about it.

I run heavier buffers in some rifles depending on my above mentioned factors. Grendel and 6.8 are different than a 223/556.

OP yes youíll need a different bolt. The carrier is the same. Mags on the 6.5 and 6.8 are different than a 223/556 as well.

Iíd lean towards a separate rifle altogether. You can make different uppers work. But for a few hundred difference you donít have to deal with it.
I have a Wilson Combat Upper 20" barrel in a Grendel, this rifle is not suppressed. The lower is a STA billet originally set up for a Wylde, should I change out the buffer? If so, what weight?
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Old 07-10-2020, 12:16 PM   #15
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Quote:
Originally Posted by Radar View Post
I have a Wilson Combat Upper 20" barrel in a Grendel, this rifle is not suppressed. The lower is a STA billet originally set up for a Wylde, should I change out the buffer? If so, what weight?

What do you have in it currently? H is always my starting point.

The interior weights can easily be swapped with tungsten or steel to augment the weight.

If you need a steel one or two PM me I can send you a couple I have collecting dust after making most all my stuff H3 for suppressor use.

0 Tungsten 3 Steel = CAR
1 Tungsten 2 Steel = H
2 Tungsten 1 Steel = H2
3 Tungsten 0 Steel = H3

This is where I buy mine from.
https://www.kakindustry.com/ar-15-pa...eight-tungsten

Last edited by BigJimmyRustler; 07-10-2020 at 12:28 PM.
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Old 07-11-2020, 11:46 AM   #16
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Quote:
Originally Posted by sir shovelhands View Post
I run 5 uppers on one SBR lower: that seems like an expensive proposition if you don't use stock triggers.

What price are you getting for lowers with a full lpk and pistol buffer?
You can easily do it for sub $250. But I'm addicted and end up spending more than that on PDW braces and nice triggers.

BUT I take large groups shooting and let them shoot my guns all at once. So having them take turns on a couple SBRs instead of a bunch of pistols and SBRs is gets annoying. And frequently they ask to shoot a certain setup and it is an upper I already removed. It might not make sense for everyone, but I got tired of switching. Not to mention when I travel out of state to shoot, pistols don't require me to inform the ATF.
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Old 07-11-2020, 09:46 PM   #17
bboswell
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The money is in the upper, having an upper with no lower is pointless
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Old 07-12-2020, 12:04 AM   #18
sir shovelhands
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Quote:
Originally Posted by ahlongslide View Post
You can easily do it for sub $250. But I'm addicted and end up spending more than that on PDW braces and nice triggers.

BUT I take large groups shooting and let them shoot my guns all at once. So having them take turns on a couple SBRs instead of a bunch of pistols and SBRs is gets annoying. And frequently they ask to shoot a certain setup and it is an upper I already removed. It might not make sense for everyone, but I got tired of switching. Not to mention when I travel out of state to shoot, pistols don't require me to inform the ATF.
That's what would make mine expensive. Plus all the lefty goodies
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Old 07-12-2020, 12:33 AM   #19
trophy8
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Quote:
Originally Posted by Radar View Post
I have a Wilson Combat Upper 20" barrel in a Grendel, this rifle is not suppressed. The lower is a STA billet originally set up for a Wylde, should I change out the buffer? If so, what weight?
Probably H2
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