Try taking a neck-sized case (NO PRIMER OR POWDER!!)and then seat a bullet to the maximum length that your magazine will accept (it will help if the neck has also been reamed). Then hold the bullet over a candle to smoke it (I find that this is more sensitive than using a marker). Make sure to smoke the entire bullet. CAREFULLY slip the dummy round into the chamber being careful not to touch the bullet on the side of the chamber. Slowly close the bolt on the dummy round. Now slowly open the bolt and withdraw the round making sure to use your finger against the case to put pressure against the ejector spring and keep the bullet away from the chamber wall. If you have no marks from the lands, then your magazine is the limiting factor on cartridge length. If the bullet has been seated deeper than when you started, check the smoked area to see if the bullet stuck slightly and pulled when you withdrew the cartridge (you should see a distinct ring on the bullet above the case mouth). If contact has been made with the lands and the bullet has been seated deeper, then set your die to seat the bullet a couple of thousandths deeper than this. If you had a ring at the base indicating that the bullet pulled slightly, gradually reseat it to cover the ring and then go a couple of thousandths deeper. Now take a fully sized case and seat a bullet to the set depth. Smoke it and carefully chamber the dummy round as above. If everything chambers without problems, you have your max cartridge length set for that gun. It actually takes less time to do than it takes to explain how to do it.