I remember there was a revision of the M&P 45 barrels back in early production (2008). It involved a minor added machine cut to the right/bottom of the barrel lug containing the feedramp. As I recall, it was a refinement intended to provide some increased clearance of the front corner of the trigger bar during unlocking/locking.
The only reason I even noticed it in my then-new M&P45 (early '08 production) was that it had been done at some point during the revisions of the SW99 barrels, and it caught my eye when examining my new M&P45.
When I asked about from someone at the company, they hadn't heard anything in particular about that revision (most go unmentioned), but he later told me he'd checked his own M&P45, apparently of earlier production, and his didn't have that little added cut.
FWIW, I didn't hear anything of any noteworthy revisions or changes in the M&P45 barrels in my last armorer recert class at the end of last year. Nor have I closely examined any of the new production M&P 45's we've recently been issuing.
I'll try to remember to pull one from inventory and look at the barrel sometime in the coming weeks, to see if anything jumps out at me as being different.
My own M&P 45 has the original barrel, with the "ringed" machine cut of the barrel ahead of the chamber, and it's been just fine for a few thousand rounds. Just because something's been revised or refined, it doesn't mean it's necessary to replace or "upgrade" earlier parts or assemblies.
The trigger bar in mine ... and the unused new, but slightly older spare/repair one I just pulled from my parts bin ... are unmarked/numbered. Plain trigger bars. I won't be surprised to find those are going to have been replaced with new ones the next time I make an order for replenishment of spare parts (making the these 'obsolete', but still serviceable in existing guns).
Also, many times it's just the result of improving and/or simplifying manufacturing methods.
I remember when I noticed there had been a change in the barrel of the new production 4566TSW's. I was comparing a barrel from a recent production 4566TSW with some new production spare barrels, and noticed the lug was different on the barrel from the gun. The full exterior diameter machined lug at the rear of the barrel had been replaced with just a partial lug in the replacement barrels.
When I had the chance to be talking to one of the company's engineers about another subject, I asked about the barrel lug change. He said it had been a minor change done to simplify barrel machining/manufacture, after it had been determined that it wouldn't adversely effect the performance and longevity of the barrel.
I was therefore unsurprised when I saw that the new barrel revision appeared in the next shipment of some new 4566TSW's we received a little later.
They're always changing, revising, improving and tweaking designs and manufacturing methods. Ditto their vendors, at times.
