[SOLD "Full Auto Vol 4 : Modification manual"]

PSA, This book is still in print and available on Amazon.

[Swapping Aluminum Receiver for Steel]

Another option is to find the older tommy "pistols". IIRC, back in the day, they were sold with a 13ish inch barrel which brought the overall length to >26". And they were sold with the vertical front grip which keeps them from being a pistol or a rifle. They're simply otherwise uncategorized "firearm" and are not NFA items. If 13.5" barrel is too much, you are allowed to put as short of a barrel as you want on this, as long as you take off the vertical foregrip.

 

https://www.gunbroker.com/item/788366782

The circa 1978 WH 1927a5 pistol I had way back then had an aluminum receiver. The steel receiver was only on the carbine version at that time.