Sayas, I do think that due to the length of the 12.5 arm, there is a better match there than with the 10.5 or 9. There is more mass and better angle. Also, I am hearing from others that the Helikon is working for them in JMW arms. So it seems that it falls on the good side of the "borderline". I am sure that your setup sounds great, because you would surely know about a mismatch if you had one. This is good info for me, and others, about this combo. When the indicators are borderline, it is very hard to make a definitive statement about matchup, one way or the other. I just tried to use the manufacturers guidelines, and some previous experience to guide my recommendation.