I'd like a short 20 rnd. mag for prone/bench shooting.  Is it lawful to shorten a 30 rnd. to suit my preference? --assuming it can be done without removing the pre ban date code.

Why not just buy some preban 20's? That way you avoid issues with the spring length.
-Steve

there's nothing wrong with altering mags as long as they still fit in the org. firearm.

I'd go with this........I have never altered mags so I can't say how easy it would be but it seems like it be far more of a pain in the ass than paying for a pre 20 rounder.

Pre-ban date code?

None of mine, 20 or 30 rounders, have a pre-ban date code.

...or are you refering to the post-ban LEO code thingy?

dude, just order one of the DPMS 10 rounders...I grabbed one for bench shooting, and it's great.  Doesn't always stay open on the last round, but that's not a big deal, it more than makes up for it by not getting hung up in the legs of my rest, or even hit the adjustment screws on the legs.  

BTW, I thing you can get these for around $15 from Brownell's or Midway if you get the dealer discount.  

[url]http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=5560&s=12962#12962[/url]

Cool! -- thanks.
Someone on the exchange board was offering 20 round'ers, but I guess that responding to a request for 1/one mag isn't worth the bother.

The fellow at MEGA(receivers),OTOH is very responsive and helpful, by the way.  I've got to get one of his $154 (with parts installed) lower receivers, ASAP.

One comment on the DPMS 10rd mags - the plastic used in their followers is not as hard as that of the "green" followers in regular mags.  I've seen (and experienced) instances where the DPMS followers actually got caught up on the bolt catch arm, sliding off the edge of the follower's face and binding - not good.  Simply put, I think DPMS should have made the back of the followers a little wider and a little harder.

I had a DPMS 10rd mag with the above symptoms, changed out the follower (with one of the std black ones I had left over after converting all of my other mags to green followers - with a little tweaking using my trusty Dremel) and all is well now!

As for bench use, a 10rd clip is the way to go.  Especially if you shoot critters where it's hilly, angling up with a higher cap mag can be a problem.

Hope that this helps.

Stay safe and shoot straight!
Jim

Question on the DPMS mags;

Can you take the base plate off or is it like those Bushmaster 10 rounders?

I would say just buy a pre-ban 20 rounder.  It is a lot easier than trying to alter a 30 rounder and potentially ruin that mag.  Plus, I don't know the legalities, but the 1994 assault weapon bill prohibits manufacture of new "high capacity" mags.  And you never know, the ATF might interpret you modifying an existing pre-ban mag to be manufacturing of a new mag.  Buying a pre-ban 20 rounder also buys you the peace of mind that you are completely legal and not in the gray area of the law.

If your looking to buy a 20 rd mag IM me . I'll sell ya one.

These are good mags for the money also.

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