I am going to show you how to make a AR-15 Lower or a 10-22 Rifle into a M249 Saw
This is my ar15 build it is a build for the front end which leaves the stock untouched so you can put a bumpfire stock on the gun.
My second project I did was to convert a 10-22 into a full saw profile. 
you start with a stock 10-22 Ruger which you can get for 150 dollars if you look around. 
Next you order a broken m249 Air-soft riffle. Make sure its the full metal version this will only work for 22 cal ammo anything above that is to powerfull for the build materials used on Air-soft riffles. I was able to find my donor m-249 on ebay for less than 100$ For the AR build you need to use GI surplus parts since your shooting 223 ammo it needs to be able to stand the test of time.
stock is 60 dollars. heat shield top and bottom is about $120. Bipod is $11. Grip is about 20 dollars and ar lowers range from 38 dollars to 250+ bu the upper is a side charging "Bear creek Arsenal" for $128 for upper and BCG. The barrel is 99 Dollars. If any one is intrested in me putting up build diagrams and detailed guide let me know. if there is enought intrest I Will. Thanks for looking and keep posted for other gun projects.
Im finishing up a 93r conversion from a 92 Beretta it requires a SBR tax stamp to do the forward grip will keep you posted.
Thanks