Well I have had this rifle for about two months. I have used the regular 10 round magazine with great success. I have a blonde wood stock on it it seems to be the regular wood stock with a different finish. I used winchester ammo and also federal. However I also posted this at marlinowners.com and they said the following:
Make sure it seated properly first. I would also take a good look at the plunger and consider replacing it. If the plunger is worn it won't hold the mag correctly and therefore not feed properly. Since the top of the mag is near identical to the standard BX-1 mag, it shouldn't be a fit issue. Maybe the added weight of the extra rounds are causing it to sag, again due to the plunger? Just some theories, I'm an owner but no expert.
I've also heard many people claiming to have problems until they dismantle it a clean out any rough areas and burrs. I'd give that a go for sure as well.
Wait a minute!? Where am I!? Oh yeah, the MARLIN forum!
Just giving you a hard time but you might have better luck over at rimfirecentral.com...plenty of Ruger experts there! Tyler Durden
I'll post the link of the part below.