You have a couple options,
you can replace the forend cap with one the has the stud, or you can get a sling swivel that clamp on to the magazine tube.

 

Really, the forearm cap option is the best, the magazine tube is made of pretty thin metal and can be easily bent. It's not seamless tubing, but just a rolled strip. Not very rigid. It's easy enough to drill the existing cap for a swivel stud, tap or silver braze it in place. Factory ones are riveted.

Numrich has had them in the past with the stud, I wouldn't count on their availability forever from that source, though. A new cap WILL take some careful fitting to get the screws to line up and still have a forearm that doesn't shuck back and forth. Inletting black or gold helps with the fitting. I use very sharp chisels and cabinetmaker's files for that job.

Stan S.

 

Here you go.

Forend Tip w/ Swivel Stud
Product No. 451400B
$25.10


http://www.gunpartscorp.com/catalog/Detail.aspx?pid=451400B&filter=marlin+forend+swivel&catid=0

 

and a second link

http://www.midwayusa.com/product/18...in-forend-tip-marlin-336a-30-30-winchester-1894-1894p-1894cp-satin-walnut-brown

 

Thanks all!

 

I bought mine through Marlin.

Way better than the feed tube clamp it had.

Didn't cost much.

 

Whatever you do.
DON'T PUT THE REAR SWIVEL STUD THROUGH THE BULLSEYE!!!
M.

 

I ordered a new cap with the swivel for mine.

 

I used the clamp on style. It works fine and I can carry the rifle lower by moving it up under the barrel band by the front sight.