Apples to oranges.  Will a 20" blaster barrel outshoot a 10.5" match grade barrel?  Probably not.  

TL/DR: it’s probably not the fault of the barrel.

You also need to consider inherent accuracy of the barrel vs. the practical accuracy of the system.  A match barrel mounted in a rigid test fixture is going to produce smaller groups than it will in a system with a bunch of moving parts.  And with the MAX-11/15, you also have the open bolt operation which gives you a much longer lock time while a lot more mass is moving around as you wait for the round to strip from the magazine and fire.  

Does that mean the system can't be accurate?  Not at all.  It's not going to shoot bug-sized groups like a benchrest rifle will, but you should expect reasonable "machine gun" accuracy.  My opinion is that the "open bolt reduced accuracy" thing that a lot of people talk about is overblown.  I have OB subguns that I can shoot more accurately than closed bolt PCCs.  But it does start to make more of a difference at rifle-caliber differences.  I find when shooting from a good rest at a bench, I get group sizes from my MAX-11/15 that are roughly double what I get from an AR15 with a similar quality barrel.  So if I'm shooting 2-4" groups at 100 yds. with the AR with blaster-grade ammo, I'm getting 4-8" groups with the MAX.  

We ran a rifle-caliber machine gun match last summer.  I was actually surprised at how poorly I did.  And I came in 2nd place out of around a dozen shooters.  Targets were between 30 and 200 yards.  All was good at the beginning but my practical accuracy dropped off a cliff.  I'm thinking I had to have been holding the gun canted, as shots were hitting a good 18" off to one side by the end.  Here's what I posted about it over at snugbus.com afterward:

Well, can you? After all, it's just like a rifle, and we can all shoot sub MOA groups at a thousand yards... offhand... in the rain... with blaster-grade ammo. Or at least, that's what all of the internet commandos lead me to believe. From which I can conclude that my abilities must be severely lacking.

It's been about a month since I set up and ran this course. I had considered not posting, since it's not exactly flattering to my shooting ability. I put my video together (watching shooting at any kind of distance isn't all that entertaining. Be forewarned). But maybe I can learn from all this. If nothing else, maybe you all can be entertained by my lack of shooting ability.

So I had posted some info at Uzitalk in the Michigan Subgun Championship thread about this (https://uzitalk.com/forums/index.php?threads/fourteenth-annual-michigan-state-subgun-competition.99105/#post-871633), but here are a few more details. I've always wanted to have a rifle-caliber machine gun. But then I start thinking about it and reality kicks in, and I have a difficult justifying the expense to myself. And I remind myself, if I owned an M16 for example, it would spend 90% of its time set up in 9mm, and another 8% in .22LR. So I should just be happy with the pistol-caliber subguns I am fortunate enough to own. I enjoy shooting subgun matches, but there isn't much out there in the way of rifle-caliber machine gun competitions. Yes, there are machine gun "shoots", where a bunch of people set up their MGs on a line and turn junk cars and appliances into bullet-riddled scrap. Don't get me wrong, those are fun too. But after a while they feel like just turning ammo into noise and brass.

Competitions seem just a bit more... directed, intentional? I don't know. I am aware of a couple "machine gun" matches. There is the annual belt-fed match in Las Vegas that I've never made it to, which also has a class for mag-fed machine guns. And I recall seeing the North TX subgun match will occasionally have a "rifle caliber" class. I keep meaning to ask the match orgainzer how he sets those up. If it's basically a subgun match utilizing mostly paper targets, with some rifle-grade steel set at a safe distance, but still shot at subgun distances.

But the idea I had was to set up a match using rifle-caliber machine guns at rifle distances. The range where we shoot the MI subgun match has bays out to 200 yards. While not anywhere close to "long range", it's still a pretty good stretch compared to distances we usually shoot subguns at. My idea was to do a small trial run with a handful of shooters, utilizing mostly the same stages we would shoot in the subgun match, with only minor changes required. The entire machine gun "match" would be run as one stage, incorporating bays 2, 1, and 6 of the subgun match, shot in that order.

Targets were all rifle-grade steel at distances from 50 to 200 yards, as well as some paper USPSA targets at about 30 yards. For the sake of safety and firearm control, any shots beyond 50 yards were required to be fired from a supported position (from a bipod, shooting bags, monopod off the mag, prone, etc). I had tables set up at the 100 and 200 yard sections to rest the gun on.

The match was set up with something like 44 shots required if you had no misses and fired no extra shots. Steel required one hit and paper required two. I figured if someone wanted to run a 100-round mag and do no reloads, that was great. Just to ensure I had enough ammo, I had planned to start with a 30-round mag at the first bay, then load to a D-60 as I moved to the second bay and finish out with that mag. I had a plan, it was all set in my head... and then the buzzer went off.

The first bay had four 6" plates at 50 yards plus three silhouettes and a 12" plate at 100 yards. I easily tagged the four small plates with four shots. And then it went downhill from there. I fired the rest of the mag at the big targets at 100 yards. I was shooting from a rest, with the gun set up exactly as I'd shot it just earlier in the week. And I was missing badly. I finally figured out my hold just as I ran out of ammo. Reloaded to the D-60 and hit the final target before moving to the paper in the middle bay. These were ok. Mostly. And then I moved to the final shooting position, which had steel at 50, 100, and 200 yards.

Now the 50 yard shots that I had nailed so easily on the first bay started to elude me. Was it because I had been moving and my heart rate was up (maybe some). Thinking back, was it because I was at an awkward position on the table with gun canted, moving my POI? Likely. But none of that crossed my mind at the time. I finally held to the right, well off the targets, and started getting hits... and promptly ran out of ammo again. I loaded to a too-long 60-round Schmeisser mag to finish things off, eventually.

In the end, I expended 143 rounds on my "simple" 44-round course. It took me 3-1/2 minutes to run through, and I had three paper targets that were not neutralized (only one hit each). So yeah, I did pretty poorly, if I do say so myself.

But I was in good company. The first place shooter ran a belt-fed HK-23 with a 200-round belt. He only had something like 30 rounds remaining at the end of his run. And the other shooters didn't do much better. I could blame the open bolt operation of my gun, but there were a number of closed-bolt guns that performed about as well.

Below is the link to the video, if you're interested.

Having posted all of that, I'm interested to see what you come up with for your MAX-11/15.  I wouldn't be opposed to putting something heavier and/or longer on the gun.  As you note, with a suppressor it can get overly long pretty quickly.

Lage makes a set of vice blocks.

What is the carry handle you have on your “heavy”?