The Firing Line Forums - View Single Post - Can a 5.45x39 upper receiver fit a 5.56 lower receiver?

pilpens · December 23, 2011, 11:10 PM

5.45x39 upper will fit a milspec lower.
The only real difference between a 5.45 vs 5.56 AR is the barrel and the bolt. Maybe, the hammer spring may need to be stronger for a 5.45 depending on ammo you use.
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My S&W 5.45x39 upper is on a Spike's tactical lower. Prior to the S&W 5.45x39 S&W, I used the lower with a BCM 5.56x45 upper.
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More than 7 rounds of 5.45 in a 20 round 5.56 magpul polymer magazine will produce a jam.
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30 round C-products SS 5.45x39 magazine is good --- came with the S&W upper.
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I like the felt recoil with the 5.45 than 5.56. It is milder and it shoots just as well as any 5.56 at 50 and 100 yards.
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Beware of corrosive ammo. My S&W bolt has pits after 2000 corrosive rounds. Must clean after range trip.
If you plan to shoot a lot of rounds per range session, 5.45 corrosive is good to save on cost. If you shoot a few rounds once or twice a week, stay away from corrosive --- too much cleaning.
Non-corrosive 5.45 steel case ammo is just as expensive as low end steel cased 5.56.
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December 23, 2011, 11:10 PM	#4
pilpens Senior Member Join Date: April 24, 2008 Location: Ohio Posts: 1,696	5.45x39 upper will fit a milspec lower. The only real difference between a 5.45 vs 5.56 AR is the barrel and the bolt. Maybe, the hammer spring may need to be stronger for a 5.45 depending on ammo you use. ============= My S&W 5.45x39 upper is on a Spike's tactical lower. Prior to the S&W 5.45x39 S&W, I used the lower with a BCM 5.56x45 upper. ============= More than 7 rounds of 5.45 in a 20 round 5.56 magpul polymer magazine will produce a jam. ============= 30 round C-products SS 5.45x39 magazine is good --- came with the S&W upper. ============= I like the felt recoil with the 5.45 than 5.56. It is milder and it shoots just as well as any 5.56 at 50 and 100 yards. ============= Beware of corrosive ammo. My S&W bolt has pits after 2000 corrosive rounds. Must clean after range trip. If you plan to shoot a lot of rounds per range session, 5.45 corrosive is good to save on cost. If you shoot a few rounds once or twice a week, stay away from corrosive --- too much cleaning. Non-corrosive 5.45 steel case ammo is just as expensive as low end steel cased 5.56. ============= Last edited by pilpens; December 23, 2011 at 11:20 PM.