Almost all kit guns, and many parts guns( there is a difference) leave extra wood so that you can finish the mortise to a particular lock plate and barrel combination. Even when a mortise is cut for a lock, its expected that the builder complete the mortise by making it as deep as necessary to bring lock plate against the barrel. The soot blackening and wood removal method has been described adequately already here.
Unfortunately, some commercially made rifles show up with a gap between the barrel and lock plate. Your choice is to return the gun, or to " fix it " by redoing the lock plate inletting properly. I read about people using Brass and lead or even tin " shim " to fill in the gaps and simply cringe. Usually the use of these devices will result in a tipped lock plate in the mortise, and other problems result with the function of the lock itself. I am also sure that some of the members here who have recommended " Shimming " have done it " right " and are experiencing no problems, YET.
If there is a weakness in the side lock design, its that the lock plate mortise leaves so little wood to support the barrel and butt stock, its easy to understand why cracks in stocks occurred in this area. I also have seen badly cocked lock plates due to some folks getting too agressive with a screw driver every time they " tighten " a lock plate bolt. I saw one gun- not very old- where the lock bolt was intering with the operation of the cock, because it was sticking out of the plate too far. When I held the rifle up to the light I could clearly see that the wood above the lock bolt had been splintered and crushed next to that bolt hole, and the plate drawn into the wood to the point that the lock would NOT function. I passed on that " deal ". I understand how such a thing happens, but really, it began with someone not understand how little wood there is under that lock plate to support it in the stock.
So, do the job right. You can cut away the wood with sharp chisels and an Exacto knife, and if you smoke the metal parts and then mark the high spots on the wood to remove, you should have no probem provided you go slow and shave wood off rather than attempt to do this in one step.
When you get the lockplate seated against the barrel correctly, there will be wood standing out from the lock plate, leaving it sitting down in a " hole ". That can be more easily removed by using sand paper and a block of wood to remove the wood evenly, down to the plate. Yes, you may have to refinish the stock, or want to, but that will also teach you about stock finishing, and let you put a much nicer finish to your rifle than anything coming out of a factory. I have seen some stocks where only the area that was reduced was restained and then refinished, and the color and finish were blended perfectly with the existing finish. Its worth trying to do. If it doesn't look right, you can go ahead and strip down the finish and redo the entire stock.
Most shooters who live long enough have a rifle that has seen more than its fair share of dinks, dents, and scratches over the years, and deserves to be refinished to show its glory and beauty again. So, the stock gets refinished, either by the owner, or by someone he hires to do it. I have refinished gunstocks for several relatives, and friends, and they have always been shocked to see how Pretty the wood in the stock is under all those years of accumulated dirt, oil, UV light dulling and yellowing the finishes, and minus those dinks, dents, and scratches. One man told me he honestly expected to see the stock without the scratches but wanted to know how I got out all the dinks and dents? ( steam) Another had a gun that the factory " painted " brown, and was in shock to see the grain of the wood underneath that brown paint when I returned his gun with the stock stained and then finished with hand rubbed oil. He told me that unless he had the serial number for the gun, he would not have recognized his own gun.
So, don't hestitate to do the job right because you may have to refinish the stock, too. That is also a great learning experience, and there is a lot of advice on how this is done down under the Gun Builder's Bench topic here on the forum. We are here to help you through all of this work, as many of have been where you are now.
Yes, I too have had to reseat a lock plate to get rid of the barrel/plate gap at the bolster, or flashpan. In doing so on one rifle, I had clearance problems result with the ramrod hole, Which I resolved by reducing the diameter of the end of the ramrod. And these problems occur with both flintlocks, and percussion locks, where, fitting the lockplate properly may result in having to bend the percussion hammer to realign it properly with the nipple!
