My favorite shotgun is my Remington 870; bought it used for $95 on 9-11-2001 and a gunsmith told me the barrel was slightly bent at 18”. He cut it off, then I added a magazine extension so that it holds 6 rounds.
A cop friend of mine asked me about the legality of that many rounds capacity (Texas Fish and Wildlife law mandates a plug to limit its total load to 3 rounds) and I reminded him that it’s not a game gun. I had another species of critter in mind when I bought it.
If you buy a shotgun that already has a short barrel, it’s legal in Texas.
You just can’t do it yourself.