Swapping BCG between 308/6.5 uppers??

[KneverKnew]

Greetings. This is my first post here. Ive had a 6.5 Creedmoor Aero Precision M5 I built some time back that has proven to be exceedingly accurate and I really like the platform. To expand my caliber options I just finished an Aero Precision 308 M5 upper build to use on the 6.5 lower. My question pertains to the use of the 6.5 bolt carrier group (and charging handle) on the 308 upper. Is there any big reason I shouldn’t swap the BCG back and forth between the two? This would not be permanent as I do intend to get a dedicated BCG for the 308 at some point. Just for right now, I’d like to shoot the 308.

The two attachments show the 6.5 Creedmore with 5.56 little brother below. The second pic shows the new 308 upper assemble on the M5 lower.

 

[Rocketvapor]

I would suggest waiting for at least a SECOND BOLT and maybe a SPARE BCG (that would be for either).

Bolt lugs will mate to the extension (at least that's what I've read).
The wait time for another bolt should only be a few days.

 

[KneverKnew]

I would suggest waiting for at least a SECOND BOLT and maybe a SPARE BCG (that would be for either).

Bolt lugs will mate to the extension (at least that's what I've read).
The wait time for another bolt should only be a few days.

Thanks for the suggestions. It will be another couple weeks before I could buy a new bolt carrier group or even a bolt. I was just wanting to take the rifle to the range to break it in before then. If this is not a good idea for the long term health of the guns accuracy then I guess I could wait.

 

[ShooterwithNoName]

When I build a new AR, whether it be 223, 308, etc. I take the new barrel, a new bolt and break them in together. This isnt necessary, maybe even OCD, but it's my method. Before I ever fire a round I cycle the brand new barrel and bolt, literally hundreds of times. I apply Ballistol to the extension and bolt and run the shit out of them by hand. Effectively mating them together until my wife freaks out on me from cycling the rifle hundreds of times.

I then clean all the lube and grime/dust/grit that builds up from doing this, re-check headspace and if all is well, fire it. But once I have done this to a rifle, I keep that bolt paired with that barrel. I considered them, mated. That said, I've swapped BCGs between rifles before, but I pull the bolt apart and run a headspace set first. I was standing 3ft away from an AR once that had an OOB discharge. It was a fluke we didn't get hurt. I've seen many swap BCGs and bolts without head spacing and it worked fine. But I cant do it, I have to know.

Good luck with your build, welcome to the hide.

 

[Rocketvapor]

I'm not sure if headspace is the only issue with mating a bolt to a barrel.
As far as I know only two measurements are needed on a bolt to verify the bolt face to lug distance.
Easy peasy to compare two bolts for headspace in the same chamber. It's the chamber part of the headspace measurement that needs either a no-go gauge, fired brass, or wishful thinking to compare two chambers. Especially moving a bolt between two different brands or calibers that might use the same size bolt.

I guess I've been lucky and never found headspace to vary between bolt to bolt more than 1 and half thou or so.
It's the non perfect alignment between the lugs in the extension and the bolt that marry a bolt fired over and over .
The force of moving a round into battery doesn't wear hardly anything. Dropping a BCG into the chamber (maybe even without a dummy round) isn't where the concern is. Firing and unlocking forces start wearing the two parts together. You can watch this happen over time by examining the surfaces of the bolt lugs that get hit hard when fired.
As good as modern manufacturing is, the mate up just won't be the same between two rifles.
A close tolerance bolt, a close tolerance extension, and a very square assembly will be close, but different.
More shots, and the mating gets better with high areas worn down. Move that bolt to another assembly and maybe the alignment will be ever so slightly different putting uneven loads on the lugs until they wear to the new pattern. That don't sound so good.

Your one and only bolt has worn in to your built 6.5. I would hold off moving it to another upper.

Or, so I've read on the interwebs

 

[JimLee]

Every upper I own has its own Bolt and BCG, seems to be a good rule of thumb.

 

[The DFC]

I'm in the every upper has its own BCG camp.

 

[308pirate]

Let's say you have one BCG for two rifles. What are the chances that one day you'll leave the BCG in the other rifle and only find out when you get to the range?

One BCG for each upper.

 

[KneverKnew]

When I build a new AR, whether it be 223, 308, etc. I take the new barrel, a new bolt and break them in together. This isnt necessary, maybe even OCD, but it's my method. Before I ever fire a round I cycle the brand new barrel and bolt, literally hundreds of times. I apply Ballistol to the extension and bolt and run the shit out of them by hand. Effectively mating them together until my wife freaks out on me from cycling the rifle hundreds of times.

I then clean all the lube and grime/dust/grit that builds up from doing this, re-check headspace and if all is well, fire it. But once I have done this to a rifle, I keep that bolt paired with that barrel. I considered them, mated. That said, I've swapped BCGs between rifles before, but I pull the bolt apart and run a headspace set first. I was standing 3ft away from an AR once that had an OOB discharge. It was a fluke we didn't get hurt. I've seen many swap BCGs and bolts without head spacing and it worked fine. But I cant do it, I have to know.

Good luck with your build, welcome to the hide.

Thanks again for the input and suggestions. This does sound like a good idea. I’ve shot the 6.5 barrel/bolt maybe 40 times. I notice it stillcycles by hand a bit “stiff” to pull open. How many rounds/ hand cycles would you say it takes before the barrel/ bolt are properly “mated”. Or are these larger systems just going to be more “stiff” due to stronger spring by nature? I’m going to try and order a new BCG ASAP. Looks like I won’t be getting to the range any time soon. Although I do a bit of shooting off my back porch from time to time. I’m just itching to shoot her. Just to see how she groups. Would ten rounds be too much for the 308 to mess up the wedding between the 6.5 bolt/barrel? If so, then I’ll wait for
The new BCG.

 

[Rocketvapor]

Stiffness comes from many sources.
You pull back against the recoil spring.
The Carrier rides the hammer back.
The bolt rings drag.
The bolt cam pin might be dragging.

Unpin the upper and cycle to see how the BCG runs in the upper.

 

[Rocketvapor]

Let's say you have one BCG for two rifles. What are the chances that one day you'll leave the BCG in the other rifle and only find out when you get to the range?

One BCG for each upper.

Story time
I've got a pair of 22 Nosler uppers (3 actually).
I can run 22 Nosler brass with a 5.56 bolt, OR run necked down 6mm Hagar brass (22 NOSGAR) with a 6.8 bolt.
Hagar brass will not chamber with a 5.56 bolt BUT 22N brass WILL chamber with a 6.8 bolt.
So, I'm doing some load work with both cases and took 2 BCGs configured with bolts for each of course.
Start off with the Nosgar loads and in the heat of the night finish up with the 22n loads across the chronograph.
Yup, you guessed it, Never changed the bolt. Both ran fine with the 6.8 bolt.
I've been more careful since then.