Steve is exactly right about a box lock being much easier. But, you can inlet a sidelock if you are determined to do a good job, and are willing to get the tools necessary. My first attempt was to inlet a sidelock into the stock of a custom muzzleloading competition rifle I was building. i used a combination of new high speed center-cutting end mills and a drill press equipped with a two axis cross slide vise. I finished the job with very small hand chisels of various shapes. This is the result. You may be able to see where screw heads bear against the wood. That is intentional.

The inlet:

[Linked Image from jpgbox.com]

The lock:

[Linked Image from jpgbox.com]

The chisels:

[Linked Image from jpgbox.com]

May God bless America and those who defend her.