Have enough guns? How about more upper receivers for your AR15?
I don’t think you even need to worry changing calibers if you want something different than a 556 upper. I’ve seen guys turn an M4 Carbine into a varmint rifle with a snap of two pins on a lower receiver. The real thing to look out for is changes in calibers, namely to a 5.45×39 upper. Many times I’ve seen guys skip the hammer spring change and end up with a light strike issue with Russian ammunition. We sold a ton of the Smith & Wesson 5.45×39 uppers a few years ago but for some reason they just don’t seem to be available.
The LWRC M6Ak was a one off novelty deal which died a quick death and no word if it’ll ever happen again.
Right now the biggest selling upper receiver we have are the Adams 5.45×9 uppers. I had a customer take apart his gun side by side to my AR15 complete upper receivers and compare the piston differences. There is a difference and I still love my LWRC M6A2 but this gun was a next best thing and a best thing for a 5.45×39 upper. You still can’t beat the cost of ammunition for these guns and the caliber does have a better reputation than the 30 caliber AK. I’ve known guys that took hits from the AK it has so much energy that it just zips through you and the Russians finally caught on that the 5.56 was a better combat caliber.