HeloDude
18 USC 922r is a pain in the ass to comprehend, but this guy probably explains it best at a caveman level.
Basically, if you make any modification to the original form of the rifle or shotgun brought into the country, it has to be 922-compliant. Those features added that are generally considered to be changing the weapon are:
- High capacity (greater than 10 round for rifles, 5 rounds for shotgun) magazines
- Pistol grip attachment
- Folding buttstock
- Muzzle device/attachment (to include a threaded barrel capable of receiving a device)
- Bayonet lugs
As stated in that link, if your rifle or shotgun incorporates those features, it no longer is considered "suitable for sporting purposes."
So under the letter of the law, such simple "modifications" as using a 10-shell magazine changes your "sporterized" Saiga shotgun into a nasty "modified" one, and you then have to replace parts until you are assured it has no more than 10 of imported parts on it.
And be careful, because not all aftermarket parts for Saigas are made in the US! To remain in compliance of 922r, the number of foreign parts must be 10 or less!
Now, whether the BATF will arrest you at the range for shooting a 10-shell magazine in your Saiga shotgun is debatable. I honestly can't recall anyone ever being prosecuted for having a non-922r compliant firearm, or the BATF inspecting weapons at a gun range; but there is always the chance and especially so if you later try to sell it.
Best to be safe for the time being and just use the 5-round mag until such a time that you can replace parts to make it 922r compliant! But that's my knowledge of the law, and I am not a lawyer nor do I profess to being one on TV. The bottom line is that it's your call...
Cheers! M2