To be more specific about this question.  I am potentially looking to get an ar conversion in the 5.45x39.  I was curious if there is a difference between the magazines made for the ar platform versus the magazines made for the ak74.  I am sure there are but wanted to ask just to be certain.

Yes, the mags are different...and finding reliable ones is somewhat difficult.

I haven't looked in a while, but ACS may be the only ones making them ATT.

There have been reports of good luck with modified PMags but I haven't tried doing so (I have a supply of ACSs and CProds).

I appreciate the info.  I am asking because I am looking at the MGI Hydra Marck-15 rifle.  From what I have seen you can do a few simple changes to convert calibers.  Wondering if it would be worth getting the 5.45x39 kit or not from a SHTF perspective.

I would say that there is little reason for a 5.45 option, now that the cheap surplus 7N7 (not sure about the designation) is now longer readily available.

I wouldn't do it.  

My nightmare.  http://www.ar15.com/forums/t_3_121/655244_Trouble_with_5_45x39_build.html

After some research, there is a list of horror stories a mile long.  Mags are the least of the worry but can be a problem.

Edit, I am now using the Pmag Gen 2 windowed mags and doing well with them.  The AK-74 mags can not work in the AR if that is what you are asking.

Edit, after looking at what the hydra is I see your point.  I think others have tried this but they never really seem to catch on.  RRA tried it but the price was just retarded.

Yeah, I can see some of the benefits of their design, but as I think more and more about things, I realize I would just complicate things. I really do like such a mindset of a modular firearm.  I think I will not pursue that route for now.  Though I still like their concept

I have built a couple of the 5.45 AR's.  It depends on what your reason is for building it, but the ammo can still be found cheap enough to warrant a 5.45 build IF you shoot enough.  

I personally think the time has passed on the 5.45 AR platform, but I am sitting on a decent stock of ammo and I built these a few years ago.

I have both ASC mags and modified magpul mags and both have worked very well for me.  My favorite are the modified magpul mags.

There have been many stories of folks that have had some issues with a 5.45 build, but I think these are the vast minority.  I have built 5 of them and had never had any major issues.  I had one that was under-gassed and had to open the gas port a bit and I also received some bolts from Ballistic Advantage that had very tight extractors, but a tiny bit of 'smithing on those and they worked flawlessly,

If you are for sure that you want to build a 5.45, no reason really not to, but just keep in mind that cost of the build when factoring in how much money you would save.  It used to be that you could save the cost of the conversion with two tins of ammo (2,160 rounds) but lately I would think that it would probably be double that.  

Still it is really fun at the range to see the look on people's face when you tell them that you are shooting a 5.45.  I have had more than a few folks try to convince me that I meant 5.56.  

can someone tell me exactly how you modified pmags to work? pics would be a plus

I love the 5.45 platform, plenty of good deals on ammo still available also. Silver Bear and Golden Tiger are my go to brands.
I've built a few uppers and other then some crap barrels I bought on the EE have had no problems. IMO, modified Pmags are your best choice. Mine run 100%, ACS and others never did. Gen 2 windowed Pmags are what I've used. You need to enhance the upward push on the round at the front of the cartridge. Take the mag apart, with a sharp razor knife lower and widen where the rounds get pushed out. I also trim back the feed lips as well. You need to cut the front anti-tilt leg off the follower. I also put a small bend in the spring for added upward pressure to the front. It sounds confusing but it's not. I love shooting 5.45, lot's of looks when I show up at  a 3gun match with it!!

Getting to be not true.  Lee and RCBS are now making dies for this caliber and you can either use 222 bras to load or get Fiocchi brass.

True, but if you reload, it would probably be better to just reload 5.56.

any pics or videos on the mods for reference?

ASC 30 round 5.45x39 mags are good to go, no issues with mine. I only have a couple of the early window'd PMAGs and didn't want to cut mine up so I tried modifying one of my 20 round straight PMAGS by cutting the legs off the follower to allow it to tilt and cutting the front lip of the mag down to allow the rounds to clear if they didn't come up all the way. It worked 90% of the time but the mag bulged enough it was a really tight fit in the magwell so I scrapped that. 20 round straight GI 5.56 mags also work well, I picked up some cheap 20s from Brownells and they work unmodified.

As stated, you can get a huldra 5.45 upper which is nice but mags are crap. I have decided to use modified follower emags and load 20 rounds then call it a day. Asc and c products mags are barely useable from my experience. Ak74 mags work and the guns are easily found.

so EMAGS work when only loaded with 20? how reliable?

My experience has been the best with EMAGS. I have tried 21-25 rds and they were reliable. Very rare to have an issue.
After I shot them while using a suppressor and they got dirty then they seemed to jam up much easier.
For me, I will not load more than 20 because the rounds will just fall out when you turn the magazine upside down.
It seems like the follower is bound at that point. Other people have said that their emags will hold more. I have not been that fortunate.
I just tried modding my ASC mags by cutting off some of the follower and I will see if that does anything, if not they are going in the trash and I will just
use the modded follower EMAGS with 20 rounds so I know they will be 100% reliable.

I read above that 20 rd aluminum 5.56 mags with black followers work. Maybe. I have none to try. If you attempt, please let me know the result. Thanks.

I've had extremely good results with my 5.45x39-specific ASC mags.

MrGunsNGear has some very reliable ones. Maybe he will send some my way. Lol.

Shot some more rounds thru the EMAGS. Those with code 13 and E3 worked with 20 rounds and the cut follower. The last one was marked 13 E2 with a cut follower and it was a truck full of fail. Not one ASC or C Products mag would go thru 25 rounds without a hangup. I even sanded and smoothed these prior. They are garbage. So in my experience the EMAGs as mentioned above are the best I have found. Everything else was not worth using. YMMV.