Hello I just got a Olympic Arms 9mm upper for my Colt Preban Lightweight Sporter and it does not mate up. The takedown pins match but I cannot charge the handle. I have 2 other uppers 1 from Oly .223 and another from DPMS .223 pre and post and they fit. Also the Oly upper in 9mm will fit on the Oly lower but it is a postban and this upper has an evil bayonet lug.
the only thing that looks like it might interfere is a block pinned into one side marked COLT.

Side note if I file off the baynet lug will it be legal to install on a post ban lower?

If you have a Colt's manufacturing model that has the "receiver block" in the lower, with WILL interfere with the pistol caliber uppers.

Re: The pistol cal conversion units from Oly have weighted carriers that are solid in the back of the carrier body. These carriers will impact the Colt's receiver block.

Solution: File off that STUPID block and be gone with it! You don;t need it and the only reason Colt's put it in their (in their infinite wisdom...) is so that you have to use a Colt's only bolt carrier that they shave the bottom off of completely to acommodate the receiver plug.

It is a solution that Colt created for a problem that Colt created. Go figure. Just like the democrats; Create a problem, and then when you come up with a solution to the social problem that you created, and charge the costs of the solution to the taxpayers, you become a "Social Engineer" and a genius in the minds of the sheep like voters. (END RANT)

Thanks for the reply, I figured as much. It doesn't even look right in there. Before I whip out the trusty dremmel I suppose lopping it off evenplane with the lower will be enough. It there an advantage to taking it out completly? I never plan to add DIAS or LL but I dont want a hole in the side of the receiver either? Do I want it all or what?

Sell the out of mil spec Colt lower and buy a preban Bushmaster lower that doesn't have that sear block.

GREAT IDEA,but I don't see many BM preban Lowers for sale,and when you do,they are Pricy..