The Barnett Raptor Pro STR has a 16.25" power stroke and is rated at 400 feet per second using a 380 grain arrow-point combination. If using field points only, you can shoot a 20" arrow (this is the shaft measurement and does not include nock and field point length). If you plan to shoot fixed broad heads you have to use 22". If you notice, Barnett tested this crossbow using a 380 grain arrow-point combination to obtain 400 feet per second, but they do not recommend shooting below a 400 arrow-point combination. They are doing this with several of their newer crossbows.
As per Barnett's 400 grain arrow-point combination formula for every 10 grains over the 400, one loses 3 feet per second. For every 10 grains under, one gains 3 feet per second. This is only achieved on a well tuned crossbow.
Weighing a 22" Barnett Head Hunter arrow (with Easton label) and 100 grain field point, this arrow weighs 434 grain.
A high quality arrow is the Barnett Custom Head Hunter (no Easton label). This is a very rigid arrow and very accurate. Barnett informed me by email in early 2017, that they built their own arrows and permitted using 20". Again this will work for field points on this crossbow.
I shoot the Barnett Ghost 410 with a 15.375" power stroke which is shorter by .87 inch than the Barnett Raptor Pro STR.
I am able to shoot 20" arrows but I must stay to the small fixed broad head, mechanical broad head or the Toxic broad head as they fit into the hollow of the riser (the area where the riser bolts to the stock). For target shooting I shoot with 20" field points which is no problem.
All the best.