I had a Manton 10/9 bore ML, it had been honed out perhaps several times and was very thin by the muzzels. I shot it with 3 dram/1.125 ounce 12 gauge loads (and "safety tested" both barrels with a 6 dram load in each barrel) and it did just fine. If it's a percussion gun it should work great on grouse, if a flintlock not so good. I say this because of the lag time between pulling the trigger and the gun going bang.
Steve
I agree with the folks who advise having the gun thoroughly checked and measured. Then, the advice to fire it with some heavy black powder loads to "proof" it will give added assurance that it won't blow up in your face. Of course, the heavy loads should be fired with the gun tied to a tire and a long string on the trigger. I would mark and measure a few spots along the barrels, and measure again after firing to check for any slight bulging that might not be apparent to a visual inspection. There's been quite a range of dram equiv. loads recommended to do this testing, so I would do a little more research as to what might constitute a black powder "proof" load in 14 ga.
Naturally, just as if you sent the gun to Britain for proof testing, it could fail and you could be left with something not fit to hang on the wall.
As to the lag time encountered when shooting a flintlock vs. a percussion gun... this is both truthful and fallacious. Truthful because this is what many folks actually experience when loading their flinters. Fallacy because a properly loaded flintlock with a good flint, good frizzen, clean and properly sized vent, dry powder and priming, and the correct amount of priming will fire almost as quickly as a percussion or breech loading gun. The difference could only be measured by a few milliseconds. When I fire my flintlocks, if there is ANY discernable delay, I know I have done something wrong. A dirty vent or too much priming are the usual suspects. When you can accept that there will be some misses caused by your loading technique, the shots that connect will be more satisfying than those you would make with a breechloader. Not knowing if the damn thing will even go off is part of the fun and sport of muzzleloading.