If it is Magnetic, It would probably be a 400 grade. Probably a 416 R
like the barrel Stainless. "BUT" There are so many options it would be wise to verify by getting an MTR (Materiel test report) from the manufacture to make sure.
I have welded some bolt handles and used 309 ss rod and not had trouble. It was suggested by someone very knowledgeable, that 308 ss
(18-8) Would be a good choice also. But it is best not to guess at the type of stainless the bolt and handle are made of.
Chances are that Remington will not give up their materials, but you may be able to get PT&G to tell you what they use.