I can occasionally only load 5 rounds in stead of 7. If I chamber a round, then I can continue loading all 7 (+1).

I know what's happening. The base of the first shell is catching on a gap between the magazine and magazine extension. How do I know?

When I place the forearm on the gun, there are about 5 threads showing where the mag extension screws in. Without the forearm on, if I put the mag extension on, it has 6 threads available. When I load it without the forearm on I can load 7 rounds every time.

I see lots of guys replacing the follower, magazine spring and/or the mag extension.

I don't think any of that is the problem. The problem is the forearm doesn't slide down far enough to allow all of the threads to be exposed for the mag extension to full seat itself flush with the magazine tube.

From what I've read, the Nordic mag extension may solve this problem. But I think I can solve it easier by trimming on the stock to allow it to slide down another 1/8th of an inch or so.

So what should I do? Try and trim on the stock (I'm not sure where to trim) or hope the Nordic extension solves the problem?

Other than this, the gun is excellent and I'm very happy with it.

Personally, I'd send it back to mossberg and let them figure it out.

You shouldn't have to buy anything and then "fix" it....that sounds stupid.

In principle I agree.

In practice, may be easier and less of a headache for me to take care of it on my own.

My time and energy are worth increasingly more to me.

Thanks good info, I am having the exact same problem and awaiting a replacement spring and follower from Mossberg, which is due in tomorrow. The gun is new (100 rounds old) and really pissing me off due to this problem, along with occasional  double feeds and sporadic FTF's with low recoil buck & slug. Maybe  I'll have Mossberg send me out a new fore grip cut to the right specs if the spring don't help,   I like the feel and potential of this gun but I may have to go back to a pump if I can't get it reliable.

would the upgraded Nordic extension fix the gap?  I would send it back but the Nordic is a thought

I refuse to buy aftermarket parts to fix Mossberg's deficiencies. I'm going to bug the hell out of Mossberg until they fix this problem at their expense.One of the reasons I bought this weapon was because it already had a extended mag tube. Although I have heard that Nordic's tube will fix this problem. Maybe Mossberg should use Nordic's tube on their SPX's.

Is the Mossberg extension a 1 piece or multi-piece part?

The Nordic is a multi piece tube that will allow you to screw the coupling (cap) down are far as it can go (whether that is 5 threads or 6) then screw in the extension tube until it bottoms out on the factory tube to avoid leaving that gap.