Most older 870 stocks would need the bolt hole reamed out to about 5/8" to accomodate the action tube on the 1100. The older stocks will also need a spacer to make up for the 1100's different lock nut position. Current mfg. stocks are supposed to fit both.
The spacer can be nothing more than a piece of 1/2 inch, sch 40 PVC pipe cut to length. I did this on a very nice 870 MC stock that I wanted to put on a 1100 Trap-T. I bored out the small hole in the 870 stock to 5/8ths and cut a piece of PVC to fit. Cut the PVC longer than necessary, then trim to fit...gkinla
Anyone who has bought a Jack West stock for a 1100 found a piece of PVC in the box for the spacer.
Check your draw bolt lenght and see if it has the5/8ths hole. If it has the shorter draw bolt sand the wider hole it will fit an 1100. If not it is one of the older stocks, and can be made to fit by taking a 5/8ths wood bit and reaming out the hole. You can then cut a piece of 1/2 in. Schedule 40 PVC to use as a sleeve for the 1100's take up bolt.