I've tried many uppers but none will fit. Seems like the old classic army uppers are different than everyone else's. The rear portion of the upper looks to have tabs that run under the inside of the lower receiver. Just need a bare bones upper to build 2nd setup for larger games. The 6 inch barrel just isn't doing it more me. Send me pictures of what you've got. I should be able to tell if it will work or not. Thank you in advance!