I am thinking about doing my first build. I want to do it in 7.62x39.

My question; is the lower receiver different from a standard 5.56 rec.?

TIA for any info.

Same lower and upper receivers, different barrel and BCG.

I have a friend that built one from Spikes and one from Rock River parts and he always had feeding issues no matter what ammo he got.

I'd do a bunch of research before you start spending money.

I know many of the mags made for 556 have feeding issues as the 7.62 rounds sometimes don't feed well out of most standard AR mags.

+1, except the carriers are identical, too - just the bolt and the barrel which are different.

As mentioned, Colt used to just provide a standard 5.56 magazine with their 7.62x39 Sporters, but you quickly learned you couldn't load them full and expect them to feed properly. The magazines also hold fewer rounds than their rated 5.56 capacity when loaded with the larger-diameter 7.62x39 cartridges.

I've never had a 7.62x39 AR but do currently have a 5.45x39 AR. It has some of the same problems with regular AR magazines and feeding. I've used C-Products 5.45x39 magazines with it, though, and those seem to feed just fine. Maybe there are similar dedicated 7.62x39 AR magazines available.

There were, and may still be, "franken-mags" which were the straight portion of a 5.56 magazine joined to the strongly curved body of an AK 7.62x39 30rd magazine, and those reportedly worked pretty well.

No different just need the 7.62x39 specific mags.

When I decided I wanted an AR in that caliber I decided to go with a lower that takes AK mags.  Solves the mag issue and I have plenty of them already, so the increased cost of the receiver was minimal.  The PSA version seems to get decent reviews.  The downside is they never seem to have just the upper and BCG in stock, so you're force to buy a complete upper basically.  Unlike the CMMG Mutant it is AR15 based, and not a weird intermediary between the AR15 and AR10.

YES & NO  It can be done both ways:

Option #1

----- 7.62x39 specific magazines that fit standard AR lower and upper is CRITICAL !!!  "C-products" brand are the current best option out there, usually SGammo.com has the best price on their 30-round capacity version and CheaperThanDirt.com is usually among the best price and availability for their 10-round version.  Other brands are out there but are generally considered inferior.  THE MAGAZINE IS THE MOST IMPORTANT PART OF GETTING 7.62x39 TO WORK WELL IN A STANDARD AR-15 PLATFORM ----- DO NOT SKIMP ON THE MAGS !!! GET THE BEST OR BE FRUSTRATED !!!
----- x39 specific AR-15 bolt head
----- 7.62x39 specific barrel
----- Extended/Enhanced x39 specific firing pin STRONGLY recommend but not required (Red X Arms).
----- ALL OTHER PARTS THE SAME AS ANY OTHER AR BUILD, bolt hold open works as normal, auto last round hold open.

Option #2

----- AR-47 specific AR lower receiver that accepts AK-47 mags
----- AR-47 specific magazine latch parts (rock & lock catch).
----- AR-47 specific AR upper receiver that is milled with larger clearances for AK-47 mags
----- AR-47 specific bolt carrier that has it's bottom milled to clear feed lips of AK-47 mags
----- x39 specific AR-15 bolt head
----- 7.62x39 specific barrel
----- Extended/Enhanced x39 specific firing pin STRONGLY recommend but not required (Red X Arms)
----- ALL OTHER PARTS THE SAME AS ANY OTHER AR BUILD, bolt hold open either deleted or only manual only.

These guns use standard AK magazines rock & lock style and are good to go but the way various venders make the necessary changes does differ slightly so sticking to one company for the first four parts on that list that start with "AR-47 specific" is recommended.  PSA Gen 2 "AR-47 specific" parts a good option.

Also worth noting that BHO won't work with AK mags.  If that's critically important to you, you're going to have to go with the AR based platform.  If it's not then the AK mag based platform is, IMHO, the easier/cheaper route...especially if you already have an AK.

And the firing pin.  My BCG came with the "enhanced" firing pin which is a tad longer to really smack the 7.62x39 primers like they owe you money.

^ nailed it.

I do.  It's a matching Bulgarian 74 that i built from a parts kit. (when there were parts kits)

Ah, well then you're starting from scratch since it's a 5.45.

Depending on how much you plan on shooting it, it still may make sense to go with the AK platform version vs the AR platform version. IIRC 7.62x39 mags are kinda expensive.

In 2019 there are better options. Options that use AK magwells and magazines that were designed for the cartridge.

The CMMG mutant is based on a shortened .308 bcg to negate the several instances of 5.56 bolts breaking due to the enlarged bolt face. It is not as light as an ar-15 based rifle.

The PSA KS47 is on gen 2. I haven't heard any complaints since they made changes, including a stiffer spring. It is basically an ar-15 upper with a modified lower which uses AK mags. It is the same weight as a basic ar. The weak bolt/face seems to have been fixed through better metalurgy in the past however many years.

Look into these before you build the same jam-omatic they have been building for 30 years.

If this is your first build, do not build a Russian 308, build 5.56
ETA: because you will have to deal with all of the above

+1 for the CMMG Mutant - Colt 9f the left, CMMG on the right: