Hey guys. Google hasn't yielded much help on this and HKPro is having similar results somehow.

Anyway, I've had this POF-5 for some time now but just recently went through the process to SBR it.

I have an HK A2 stock and an ATI F-style A3 stock. The former will go in with a hammer and the latter requires a punch in the pin with a hammer to drive it home. Hardly "push pin".

I thought it was something to do with the stocks because the butt cap goes on just fine. The thing is, if I take out the front pin and put in the rear with the stock, it goes in just fine. Then it's the front pin that has trouble going in.

Any ideas what could be causing this?

I keep a rubber/nylon hammer with my POF just for stock installation and removal :P  You could take some sandpaper and very lightly sand the inside of the A2 stock where it mates with the receiver, and even more lightly the inside of the pin holes on the A3F stock. Emphasis on very lightly.

Haha that's exactly what I've been using. I'll try to see what some light sanding can do for me.

Sometimes the receiver is a *tad* long and prevents the stock hole from making it forward to the receiver hole.

A little judicious belt sanding to trim the OAL of the back of the receiver may be in your future.

I'm not that confident in my grinding to not mess up the receiver, especially when it means $1450 + $200 tax stamp out the window.

Could a new lower housing help? If so, which ones are even remotely compatible with the POF-5?

Don't grind.  Very, very bad advice. The POF receiver is in spec, it just is a little heavy on the paint.  Put the end cap back on it, then put tape up to edge of the end cap.  Remove the end cap, then take some fine sand paper and sand the paint down a little in the area under the end cap.  Since you're only sanding in the area under the stock or end cap and not anywhere else, you won't see it after you're done.  If you go down to metal, use some cold blue to protect the metal.  When you're done, a little very thin coat of grease or oil will help get the end cap on easier.

It isn't unusual for a few taps with a mallet to get the stock on if the stock is new.  The HK armorer's manual covers this and offers this specific advice.  Over time and use, the stock socket will loose a little.

Don't waste your time with the ATI stock, since it probably won't fit.  The ATI stock doesn't fit well on my POF, won't fit on my HK94, and doesn't fit on my MKE without hammering on it.  It does fit on my friend's SW9, so I gave it to him.  The POF A2 stocks from RTG fit perfectly, so do the HK factory stocks.  My own POF SBR has a HK A3 stock and it works great.  The ATI is cheap for a reason.

As far as lower housings, basically all regular MP5 trigger group housings work on the POF.  You just have to relieve a little material for the shelf with a file (NOT a Dremel) and use a semi-auto trigger pack to keep legal unless you have a registered sear.

Awesome. Thanks for the advice, interfan! I'll give that a go.

I didn't even think about it being a breaking in issue. That could be it too.

And I kind of figured it would be a "you get what you pay for" situation with the ATI, but I still held out hope. Maybe when I get a little more funds to blow on a stock I'll try out a more authentic collapsing stock. Lesson learned.