My barrel twist rate is lower than the optimum recommended, will this be a problem?

No. As long as your SG (Gyroscopic Stability) factor is above 1.05. Note the formula has an accuracy to +/- 0.1 SG. Numbers below 1.05 will lose both Accuracy and BC, and most likely tumble. Numbers between 1.05 and 1.45 will have a lower BC but retain Accuracy. Stability levels above 1.5 will have optimum Accuracy and BC. For every 0.1 SG you drop below 1.5 you will have an average of 3% BC loss. This number is better and worse for certain bullets. But for example, if you calculate out to a 1.3 SG then you can expect an average BC loss of 6%. On the other side, over spinning a bullet is a bit of a myth. So if your barrel twist rate is far faster than recommended, you only need to make sure you account for the added spin drift. Also remember the recommended twist rates are for EVERYONE. So shooters at higher altitude, can get away with slower twist rates. Your DA (Density Altitude) plays a role here.