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arakboss

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This idea came to me tonight while I was thinking about which Big Bore AR upper to buy.

Here is my idea. I want to take a standard upper receiver and dremel out the ejection port to allow for big bore cases to eject. Then I want to attach a big bore barrel to the upper receiver with a free float barrel nut, hand tight no vice block, no torque wrench, etc. Then I will install a free float hand guard with anti-rotation tabs. Install will now be complete. To switch barrels I will remove handguard, remove barrel nut (hopefully by hand) and install different big bore barrel, handtight only. Then reinstall hand guard. The uppers will be set up for single shot use, no gas tube.

Here is why I think this will be safe. I bought my second AR used and the seller had installed a midwest industries quad rail on it. The barrel nut was the old school type that had to be indexed. The quad rail hand guard threaded on to barrel nut and had a locking ring that tightened up the handguard. After firing the rifle I noticed the barrel would rock ever so slightly inside the hand guard. I thought the hand guard was loose and tried tightening it. It turned out the barrel nut was finger loose but the gas tube kept it from becoming any looser. I fired at least 100 rounds before discovering the problem. I am guessing the barrel was not loose enough to throw off headspace and it wasn't going to come off all those receiver threads. That is why I think a hand tight fit will be safe and the hand guard anti rotation tabs should keep barrel from loosening once hand guard is mounted.

If you think I will blow my face off with this system please explain why, in detail.
 

osprey

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It would probably be fine but why not at least take the extra few minutes to snug up a bit past hand tight with a barrel nut wrench. Accuracy may be improved by having a barrel nut closer to min torque spec. With different barrel you may also have to have different shims to get nut oriented to accept handguard to proper position.
 

Lilhigbee

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TNW already has a QC barrel setup. Jeannie won the OFF raffle of a TNW AR a few years ago and it has it. If you are familiar with the cam-locks that are used on fire hoses, the barrel retention works the same way. Pop the cam-locks open, pull out the old barrel, insert the new barrel and close the cam-locks. Done!! This is a piston gun, but DI could be done the same way. Change calibers in seconds if they use the same bolt face, or change bolts too and go crazy.

ETA; It looks like they've discontinued it but here is their listing;
https://www.tnwfirearms.com/product-p/sgpx-cplc-xxxx-xxxx-xxxx.htm
 

arakboss

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It would probably be fine but why not at least take the extra few minutes to snug up a bit past hand tight with a barrel nut wrench. Accuracy may be improved by having a barrel nut closer to min torque spec. With different barrel you may also have to have different shims to get nut oriented to accept handguard to proper position.
I could pack a barrel wrench in the bag just for a little extra security. The barrel nut I am going to use doesn't require any indexing. The hand guard is a clamp on style as long as the hand guard clamp doesn't loosen it should allow the anti-rotation tabs to keep the barrel from moving. I doubt there is much force trying to turn the barrel during firing.
 
Personally I will just use a complete spare upper, much faster than messing with a handguard. On a piston actuated action it might be easier?
 

arakboss

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What would happen to the point of aim/impact in relation to the sights or optics every time it’s swapped?
Good question. A change to a different caliber barrel would require optic adjustments but changing to a different upper would as well. If I was to take the same barrel off and put it back on, would the aim/point of impact change? Maybe.
 

monkeybutler

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I’ve certainly never seen drift in the dimensions of upper receivers or antirotation tabs. They all fit perfectly together without any gaps. o_O

Does your health coverage include facial reconstruction?
 

arakboss

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From the comments so far it looks like the main concern would be whether there would be an impact shift from mounting and remounting of a particular barrel. My experience with sharing an optic and lower receiver across multiple uppers is that optic adjustments are a given when switching uppers. I wouldn't expect that to be much different when switching barrels in a single upper.

As for buying or dremeling multiple big bore uppers vs changing barrels. It's a cost issue. I am a cheap dastard and in order to mount three barrels permanently I will need three uppers and three compatible barrel nuts at a mininum. I could then share a hand guard between the three. For me there is some fun value in experimenting with barrel change idea. I don't want to blow my face or hands off so I will give the idea some more thought before buying the big bore barrels.

I can experiment with a currently owned 5.56 barrel to see if the barrel will stay tight between firings.

If anybody spots a great deal on in stock big bore uppers let me know.
 
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The problem I have with this is it will only take one lapse in judgement out of a thousand "barrel changes" to make a mistake you can't unmake and the medical bills would likely cost more than a couple extra stripped uppers and nuts. I understand the idea, however I wouldn't ever consider not giving the nut just a bit of snugging past have tight as if you have any wiggle, you'll slowly start making the aluminum bits more egg shaped than round. Clearances start to change, you get lazy and barely snug the barrel nut, get a hot round in there and pop goes the weasel.
 

Lilhigbee

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I would be curious to see an AR fired without a barrel nut on it. I bet the results would be less than spectacular and probably downright dull.
 
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If you have a high quality scope/optic mount, repeatability between uppers would not be an issue. Although the zero might be different, you could easily have notes on the correct zero for each upper.

I'm am in the uppers are pretty cheap camp on this one.
 

kmk1012

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If you have a high quality scope/optic mount, repeatability between uppers would not be an issue. Although the zero might be different, you could easily have notes on the correct zero for each upper.

I'm am in the uppers are pretty cheap camp on this one.
We are talking about swapping barrels, the optic attaches to the upper receiver. I’m in the understanding that swapping a barrel would move the zero, possibly a great deal in between changes. Aluminum threads in the upper would also rapidly wear out as well as the hole machined for the barrel to fit snugly into.
 

Ura-Ki

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You could use the Winchester interrupted thread design and locking tab to make a take down/barrel swap rifle that actually works!
Winchester actually copied Dahlgren with this idea, the interrupted thread idea goes back to Dahlgren's breach block locking system for canon, it is very strong, and very safe!!
 
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