With the cost of 5.56, some people have run 5.45×39 AR15s as an alternative. I have owned both a S&W M&P15R and a Spikes Tactical upper in 5.45.
The main, and potentially only, benefit of having a 5.45 AR15 is cheap ammo. 5.45 comes into this country in batches so when it is available it is cheap but it is not always available. 5.45 might be a good choice if you like to shoot a high volume, and can afford to make a bulk purchase of ammo early on. If you intend to just pick up ammo when you need it, 5.45 will be a poor choice.
There are many cons to running 5.45. First is that the majority of the cheap 5.45 is corrosive. Second is magazine and trigger requirements. Another is the performance of 5.45.
I’ve read more then a few sources talk about how they shoot thousands of rounds of 5.45 and have no rust issues. Most of the writers that say this live and shoot in the desert. Since I have started shooting 5.45 I have found rust on melonited barrels, rust and pits in nickle boron coated bolts, rust on chromed firing pin, rust on a Diamondbond coated bolt carrier, etc. So what does this mean? It means if you have a 5.45 AR, you are going to have to clean it.
A barrel and bolt are not the only parts you will need to run 5.45. 5.45 will run best from magazines designed to feed it. C-Products makes these magazines and I have had good luck with them. Some people report limited sucess with Lancer mags, HK mags, and 20 round GI mags, but nothing other then dedicated 5.45 magazines have worked for me. My C-products mags have sharp edges on the feed lips and have all warped from use, but they still feed and work reliably. Combloc ammo will have hard primers. A standard AR15/M16 fire control group may not net you complete reliability. S&W sold their rifles and uppers with a heavier hammer and disconnector spring due to this issue. This did make for a heavy trigger pull. Some match triggers will reliably fire this ammunition, I use a Geisselle trigger in the rifle I shoot 5.45 in.
For years I have read people claim how 5.45 is some sort of magical armor piercing “poison bullet”. In reality its a moderately performing round, moderate in both accuracy and stopping power. While not a terrible round it falls behind most any modern high performance rifle bullet design. 5.45 may be capable of interesting things as a cartridge, don’t expect much from the bulk packed 40 year old commie ammo.
If you are lured by the low cost of 5.45 ammo, make sure to factor in the costs of a 5.45 upper, mags, and trigger parts. You will most likely need to use at least 4000 rounds of 5.45 before you reach a break even point the money spent. I would only recommend getting a 5.45 if you can purchased many thousands of rounds of ammo when it is available cheap. When I got into 5.45, I purchased fifteen thousand rounds for it. Since then, the price of the ammo has gone up, and twice it was unavailable. If your considering 5.45 for the cheap ammo, make sure to buy it cheap, and stack it deep.