Here's my situation

I have a couple dedicated pistol lowers but recently traded my only rifle for an AK, so I am planning on building a rifle lower with either a x39 or 300 upper. I also have been waiting for the PSA 10.5" Upper to go back on sale.

Today I came across a deal for a new M&P15 Sport for $580 out the door.

I was thinking about just using the lower to start my rifle build and then just swapping out the barrel on the upper with a 10.5" from Mas Defense and calling it good.

Would I be able to reuse the carbine gas tube and FSB with the new barrel since it is a carbine length as well or could it possibly be pinned on and not suitable for reuse after removal?

I might shoot the rifle a couple times as is just for the heck of it but it's main purpose would be to break down and use for the separate builds. Does what I'm thinking even make sense, or would it be better to just build the lower from scratch? Most of my time behind the rifle will be at the range but I might be able to go on my first hog hunt in November so I want to have something ready just in case.

Personally I'd highly consider just cutting down the M&P barrel and getting it threaded, much cheaper than a barrel and the M&P is a generally a good one to begin with.

I didn't even think about that. The barrel has a 1x9 twist, do you think that would be an issue at a shorter length?

I would not hog hunt with a 223/556 and if you do, you need a heavier bullet which the 1x9 won't stabilize. If your going to hunt pigs, be pragmatic and use an effective round.

Your best option in a short action AR is a 6.8 with 120gr SST bullets, but 6.8 might be too expensive to shoot as a range toy for some people.  7.62X39 is effective from a short barrel and next best is 300blk IMO. You can get just under 2k fps with a 110gr bullet through a 8" barrel. You can drop a pig with 223/556 with a well placed round, but who wants to take that chance?

These are just my opinions, but hunting for over 40 years taught me to use the proper weapon to drop game in their tracks.

Hi your best bet it to go with the MAS defence barrel. You can reuse the gas tube and most parts. But not the Front sight base. If you cut the 16in barrel down. You would need to ream out the gas port hole to make it work properly. I would go with a clamp on type front sight base or a low profile gas block. And FF rail. And sell the compete M&P barrel on the EE. In the end it would be cheaper to just build the upper you want. If you have the tools.