Hi and welcome to the forum! Barrel length and accuracy are tied together with a twist rate and the specific load/projectile used. A longer barrel is not always more or less accurate. The other factors, like chamber dimensions, mentioned twist rate, bullet weight, and more are going to all play a part.
What is the true advantage then of barrel length?
Longer barrels afford more velocity.
Shorter barrels are easier in tight spaces.
Again Welcome from NE Ohio!

 

Short answer: no.

Longer answer: accuracy is more a function of shooter ability than firearm construction.

 

Gerarddm...Welcome Aboard...from NE Indiana!
In short, in 5.56 / .223, not necessarily!
For my needs, I tend to prefer a 16" barrel over an 18" barrel for ease of use!

 

All things being equal other than length, on a man size target at 100 yds or less with a mini you probably won’t know the difference. But with a more accurate gun with open sights the longer barrel will generally give you an edge because the gun has a longer sight radius.

 

One thing not mentioned yet (and you likely already know this) is that shorter barrels are obviously lighter weight so quicker handling. More jittery, the way I look at it. Harder to hold on target. That's why dedicated target rifles have long, thick barrels.
Probably won't make much difference in 2.5 inches.

I like long barrels for velocity and weight so my 6.5 Grendel AR and M-16 clone both have 20" barrels. But my 10mm AR carbine has a 16" for quicker handling.

 

If they’re both recent vintage guns the accuracy is going to be pretty close to each other. The Mini accuracy is average and in most cases an AR platform will shoot a bit more accurate.

 

For the average shooter, the difference will be negligible.

 

I do not disagree with the comments above, which cite the various factors pertaining to accuracy. However, left’ get down to basics. Barrel length is a factor of velocity. Too short a barrel will mean the bullet will leave the muzzle before all the powder is burned. Too long a barrel can result in a bullet that clears the barrel too late to get the advantage of the full power burn. The perfect barrel length is one that the maximum burn is achieved as the bullet exits the muzzle.

Barrel length affects velocity, which in turn is a physical factor in achieving accuracy. So barrel length can affect accuracy in a positive or negative manner. All that said, at 100 yards I would go with the 16 inch barrel because of handling. The more rifle you handle the more it can affect accuracy.

 

To add to @dweis posted. If you have two identical barrels, only differing by length, a shorter barrel is stiffer.

 

No.

 

Answer to original question is kinda complicated and has no simple answer, except that shorter barrels are "handier" and easier to fit into small spaces, such as vehicles. But in that instance, folding stocks are a better answer.

Yes, some folks have "chopped" their Mini barrels and have obtained better accuracy.

I kinda question this, since many folks, by chopping their barrels "may" have eliminated damaged muzzles. A simple and DIY re-crowning of damaged muzzles "might" have been far less expensive. Who knows?

Also, we now have decent "Barrel-Struts" for Pencil-Barrels which work very well in increasing accuracy with such Early barrels. Dual-clamp Barrel Struts FAR more effective than Single-Strut units, as per Mfr. Maybe not so much on any later "Heavy Barrel" Mini.

Latest iterations of Minis have more "meat" on their barrels than earlier versions and are more accurate, overall.

Longer barrels deliver more energy behind the bullet, thus increasing range.

With respect to common M-193 55Gn bullets, the bullet velocity when impacting soft tissue makes ALL the difference between bullet yawing/fragmenting within the target and simply punching .223 holes.

All this is aside from other more fundamental factors, such as the all-important fit of the barreled action to the stock. Most often this can be significantly improved for almost any Mini.

To directly answer OP's question, I think I'd take the longer barrel. At least you have options then.

 

A decent shooter would probably not be able see a difference between a 16.5 or an 18 inch barrel. And it would also depend on how well the rifle is tuned in.