I will try and be brief.......the Romanian AKM (fixed stock) and AKMS (underfolding stock) are about as close as you will get to a Soviet-era AKM pattern rifle, they are built by RomArm at the Cugir Arsenal on ex-Soviet machines.The bolt and bolt carrier are built to AKM-pattern dimensions, have a 45 degree gas block (obviously not functional on ours) and they will accept anything from a 10 round mag to a 75 round ChiCom drum as they have a standard military AKM-pattern feed ramp.The standard Romanian hardwood furniture ain't the best but the good news is that standard Soviet laminate furniture will fit straight on, as will the classic AKM slant muzzle brake.You can even fit AR type tat if you so desire but it is just wrong on so many levels!
The Saigas have the fact they are built at JSC Kalashnikov Concern (formerly Izhmash) going for them.They are closer to an AK-74/AK-100 series rifle in that the bolt and bolt carrier are AK-74 pattern (smaller dimensionally than their AKM counterparts and not interchangeable), they have a 90 degree gas block and like their AK-100 series counterparts they use a "third rivet" on the LHS of the front trunnion to start turning the bolt into battery.
AK-103 spec Saiga that I had with side folding stock and milspec 103 muzzle brake.......sadly no mag well "dimples",
Saigas are sold as a "sporting" rifle and as such they do not have a feed ramp on the front trunnion, they have a ramp incorporated into the Saiga specific magazines....the rear locking lug on a Saiga mag is also smaller than that of a milspec mag.Pic below show difference between a Saiga mag and a milspec mag, Saiga mag is on the left......
Some Saigas have "military" receivers that feature the classic mag well "dimples" and have milspec muzzle brakes fitted and some don't.....some have a brake and no dimples or no dimples and no brake.The most original would be one with both dimples and a muzzle brake but it is luck of the draw with what is fitted sometimes.
Feed ramps are available......
http://www.dinzagarms.com/saiga_762x39/x39guides.html
If you can drill and tap a hole they are a doddle to fit, and with a bit of filing on the mag catch you will be good to go with the plethora of military AK mags that are available.As others have said you can modify a standard AK mag to fit a Saiga but try welding an iconic Soviet Bakelite mag!
Optics.....POSP sidemount scopes, sidemount red-dot sights like the Kobra, PK-A or PK-01v etc or a 1pn58 NV scope......or use the ironsights the Tovarisch Kalashnikov designed it with!!!
Have a look here for Soviet/Russian/Belarusian optics......
http://russianoptics.net/
Alternatively stick a Capitalist Weaver/picatinny thingabob on and fit any old Imperialist tat on it
Ammo should be good old Soviet-era milsurp, preferably in spam cans so you get that hiss of Soviet-ness as you open it......and only every 7.62x39 as a 5.56/.223 AK is wrong and every time someone buys one in 5.56/.223
they make little Vassily cry!!!.
Haven't mentioned the Bulgarian offerings that Oleg sells as I haven't played with one (unlike the Romanian and Saiga variants that I have had a "few" of), but if they are as good as the Bulgarian military versions then they are very good.Built on milled receivers as well.....shame he doesn't sell a full length fixed stocked 7.62x39 version as it would make a good base for a Type 2 AK-47 replica.
Forgot to say that between a Romy and a Saiga the Saiga is a little bit ahead with regard to finish, the Romy variants are a bit more "agricultural" but accuracy-wise nothing in it and after owning a few of each I would take the Romanian.......my AKM has had thousands of rounds of dirty Soviet milsurp through it, it has got rained on all day at Sennybridge loads of times but is still as good as the day I bought it and never had a problem with it.
