I have a newly purchased PTR-91 and also have a SW MP-5 clone. The PTR has a polymer lower and the MP-5 has a metal lower. I'd like to put the polymer lower on the MP-5 and the metal one on the PTR. I swapped them as-is and the polymer seems to function correctly on the MP-5 but the metal won't work on the PTR, as you are only able to charge it half way. I'm guessing it is an issue with the extractors as they definitely are different. Would it be possible to swap out the trigger packs between the two? Are trigger packs interchangeable between metal & polymer lowers? I do know that different safeties are needed but if it's possible to swap out the packs could I keep the safeties and reuse them in their correct lower? if this is possible, is there some video or tutorial that would show how to do it?

--Thanks

The tang on the 91 housing is probably longer and may require trimming to fit the shorter rear section of the SW5.  Otherwise keep the selectors in their respective housings and simply change the ejectors, that lever like thingy on the left side of the pack. It's easy to remove them you simply push out the pivot pin and it comes right out.

The hammer springs are different between pistol caliber and rifle caliber HK's but lots of people run a rifle spring in everything, YMMV. Basically what I'm trying to say is that if there is a pistol type hammer spring in your 9mm pack it may not function in the 308 gun.

MP5 and G3 pattern (the PTR is G3 pattern) lowers are a little different.  To simplify it, the MP5 only should have one hole on the tail end that is covered by the stock socket, and this tail should be shorter.   The MP5 stock is held on the receiver with one pin, the G3 with two.  It is common to modify a G3 pattern lower to fit an MP5 by cutting off the tail a little to fit flush with the back of the receiver.  An MP5 lower will fit on a G3, but without the second hole can rattle a bit.  The safeties are different (as you observed) between the polymer and metal lowers.  

For the trigger packs if both are SEF style, not ambi, you can swap them between MP5 and G3 by changing the ejector and the hammer spring.  The other parts are basically the same.  If the polymer is an ambi version, you can't drop it into a metal lower because there are some differences in orientation of the safeties.

To swap trigger packs, assuming both are SEF style, just remove the safety by pushing it to be perpendicular with the top of the lower and push it out.  Easy.  Peruse either RTG or HKParts to see what it looks like with the safety out and it becomes pretty easy to see how it goes together.

If it is an ambi version, good info here: http://www.ar15.com/forums/t_6_9/241315_How_to__Field_Strip_and_Replace_Extractor_and_Ejector_Lever__tons_of_pics__UPDATE_Pg_1.html

Useful info:MP5 Armorer's Manual (PDF)
MP5 User Manual (PDF)

There are PDFs of the G3 manuals, too.  Google them.

ETA: You may also have some fitting to do between the SW5 and the PTR due to the size of the semi-auto shelf on the back of the magwell.  There's not much uniformity in size between clone manufacturers so there may be some slight dimensional differences there, too.  Short work with a small file.

Pics of how a metal SEF lower looks stripped are here:RTG

OK, I got the packs out and tried to swap them as a whole.....it won't work as I can't seem to line the selector holes up exactly to the holes in the lower. I see how the ejector comes out so it looks like I'll keep the packs in their original lowers and swap the ejectors out. Regarding the hammer springs, they feel like they are exactly the same tension to me. Should there be a discernible difference between a pistol and rifle hammer spring? My MP-5 clone is from Special Weapons and is probably 10 years old. Could it be possible that the rifle hammer spring could have been installed from the beginning from the factory? In the photo below, the lower on the left is from the MP-5 and that's what I'd like to install on the PTR. Will the fact that the hole is cut open be an issue if that goes on the PTR? I've seen the holes cut like that on other MP-5 clone lowers, why is that done and not left as an entire hole like on the polymer lower?

No problem. If it fits it's GTG.

Why the packs won't fit I don't know. They should swap out. Since they're both SEF.

I found a semi SEF pack which was thicker than normal so that might be why it won't.

Most MP5 packs have the open tang.

A single hole tang is one that had one of the 2 holes cut.

Rifle hammer spring is much stronger than pistol. The SW spring might be a rifle spring.

It is highly likely that SW used rifle springs in the first place.  G3 parts are cheap and lots of them are common with the MP5.  Some aren't, but get used anyway. The trigger pack (with the exception of the ejector and hammer spring) is basically the same for many HK models.

Your PTR lower has been trimmed.  A normal one looks like this with two holes in for the stock:



Having the hole cut open isn't an issue.  Factory metal G3 lowers look like this:

IIRC an uncompressed the rifle spring is shorter than a pistol spring.

Sometimes clone trigger packs and housings don't play well with others. Often due to a weld being too high and not dressed down enough on the side, but it could be dimensions at the bottom where it sits on the tabs. Could be several things. Plastic SEF has long-axle safety; steel SEF has short-axle safety.





There's no problem using a shortened tang housing on a G3, unless you buttstroke somebody and your stock bends; then it was a poor choice.





I have cut down 308 springs to an intermediate length/strength, where the pack works on everything. (I just swap ejectors.) It's easy with both factory springs to compare, but I don't recall dimensions. Check hkpro boards

 

So as it stands now, I could just swap out the extractors and I'll most likely be GTG? At some point, I'll get the correct SEF marked clipped & pinned lowers. But for now this is a starting point.

Basically, yes.

OK, thanks. Maybe I'll get to it this weekend.

No you swap EJECTORS not extractors. The extractor is in the bolt, ejector is in the trigger pack.