Now that nitride barrels are becoming a thing for ARs, does anyone make a polygonally rifled barrel (like a glock barrel), and would there be any benefit to that?

Columbia River Arms
Columbia River Arms has established a prestigious and unprecedented benchmark for excellence in barrel manufacturing with our proprietary rifling and stainless steel. Caudle 3-lan polygonal rifling consists of a series of arcs, which is held in very high regard for its accuracy, increased velocity, reduced gas cutting, and ease of cleaning.  We only use a tightly controlled blend of fully-vetted and certified, F-Bar rated, BOSS 416R steel, recognized as the finest ordnance steel available.

Columbia River Arms was BlackHole Weaponry they will make you a custom barrel you can choose 7" to 24"length, twist, thread, 30 AR15 & AR10 caliber's, fluting, & finish. They do not nitride there barrels so you would have to use a place like blacknitride.com  for another $100 to $150

Several barrel makers are offering 5R rifled nitride barrels.

My first AR was a Gen 1 M&P15 Sport. It had a 5R polygonal barrel and was EXCELLENT. For what I paid new for that gun, I believe the barrel alone was worth half the cost.

Faxon What and Why 5R Rifling?
5R is an improvement over conventional rifling by changing the shape and configuration of the lands and grooves.

No polygonal rifling is not the same as 5r. Polygonal rifling the lands and grooves are replaced with hills and valley's. 5r barrels are simply barrels where instead of the lands and grooves being at 90deg angles they have an angled shoulder giving it more surface area to grip the bullet and also in my opinion less copper fouling as there not so many sharp angles to get stuff hung up in.
That said there is not necessarily any barrel type that is better than the other and there is a lot of misconceived notions on the different rifling and there benefits and such. Hybrid 5R or 5R with angled lands but few have completely rounded lands. There are also female poly and male poly shapes. "The term "polygonal rifling" is fairly general, and different manufacturers employ varying polygonal rifling profiles. Polygonal Rifled Barrel benefits include increased accuracy, reduced cleaning, increased barrel life, and increased velocity. This is the rifling of choice on world famous sniper systems like the H&K PSG1 and MSG-90. Columbia River Arms(BlackHole Weaponry), Lothar/Walter, &  Noveske offers polygonal AR barrels.
The More You Know: Polygonal Rifling



Lothar-Walther .223 Wylde Match Grade Barrel – LiFe HD®
16in or 18in True Match Grade Barrel
Proprietary LiFe HD® Treatment Process
.223 Wylde (can shoot .223 or 5.56)
Proprietary LW-50 Stainless Steel
16" SOCOM Weighs 2lb 1oz, 16" Mid Weight is 1lb 13oz, 18in Light Medium is 2lb, 18in Medium Rifle is 2lb 6oz
All barrels Are Mid Length Gas Except for Medium Rifle Which Is Rifle Length
Polygonal Rifling
Extended/M4 Feed Ramps

The new LiFe HD® (Lithium & Iron) process is a proprietary process similar to Melonite/QPQ, but with better options. The LiFe HD® process creates a smoother (less porosity) surface finish, this imparts higher corrosion (twice the corrosion resistance as QPQ) resistance and lubricity characteristics. The hardness and depth are almost identical to QPQ, but LiFe HD® accomplishes it’s goal with a single one step procedure compared to the multi-step processes of QPQ (quench polish quench). A more uniform/deeper black finish also seems to be a byproduct of the process.

These barrels offer the best in every aspect of performance. The stainless steel allows for seamless machining with a nearly perfect bore and chamber, the polygonal rifling offers less bullet deformation and increased velocity, and the proprietary steel composition greatly increases barrel life, durability and corrosion protection.

This is what I'm talking about... I don't know why it isn't utilized more

Schneider barrels are closer to Poly rifling than most.  

I have one on a 300 win and it's great...but it's not more accurate than conventional rifling.

Most barrels are button rifled, even Larue barrels. Poly rifling is usually in odd lands like 5r or 3r.  Which 3r are supposed to be the most accurate. 3r are Black Hole Weaponry (Columbia river arms),  or one other that cant be mentioned here. Otherwise, 5r a few manufacturers have and those are very accurate. Faxon is probably your most reasonable priced manufacturer on that in their new match barrels.

http://www.customar15.net/product/lothar-walther-223-wylde-match-grade-barrel-life-hd/

I picked one of these up earlier this year.  Haven't gotten around to building the upper yet, so no actual experience with the barrel.