I recently bought a .40 barrel and a couple of .40 S&W mags to convert my 10mm Witness. I went to my range and before I swapped barrels, I loaded up a 10mm mag with .40 S&W and shot them thru the 10mm pistol. they functioned perfectly! I shot a bunch more and then swapped barrels and mags and they shot just as good.even with a 20# spring!  ???the .40 cartridge goes all the way into the 10mm chamber but I still had 100% ignition. my question is: did I waste money buying a .40 bbl. & mags? is there a reason why I should not shoot .40 in a 10mm bbl.? I know that it is possible to get a lead buildup in a .357 mag. chamber from shooting a lot of soft cast lead .38 spl bullets but all my 10mm/.40 bullets are either jacketed or plated. and the .40 case still seems to be headspacing where it is supposed to. I kinda like being able to just be able to use either cartridge as  interchangeably as a .38/.357 revolver. tomorrow i am going to do some more testing for reliability. does anyone know of a reason I should only shoot the .40 ammo in in the .40 mags. & bbl.?

is there a reason why I should not shoot .40 in a 10mm bbl.?

Since you already have a .40 barrel the question is really moot...

But, the .40 will not seat properly in the 10mm chamber as the case length is what determines how it seats. The .40 in a 10mm barrel is being held by the extractor to properly position it and there may be more stress on that part. ( not to mention possible wear on the chamber case ridge )

That being said, I have run a few 40s in my glock 10mm and it worked fine but I wouldn't make it a habit.

Let me explain this, the 1/8" difference in casing length doesn't allow the 40S&W to properly headspace in a 10mm chamber. What happens is it is solely dependant on the extractor to hold the casing in place as the striker or firing pin strikes the primer. The bullet has to jump across the transition of the end of the chamber cut (which has a sharper edge) and can and does slice off bullet material so that bullet material can roll up and build up, plus carbon and powder fouling also makes the area dirty. This requires a good cleaning before any 10mm ammo is used as they can get stuck or not chamber as a result. There is always a possibility that the cartridge can get dislodged from the extractor and enter the chamber and it will not be removed by cycling the slide.
Yes some guns will hold the cartridge securely enough to allow it to fire the cartridges with a fair amount of reliability! However if you don't inspect and clean immediately the use of proper 10mm ammo can be a serious issue as a result of the fouling!
If the 10mm cartridge is used prior to proper cleaning, they can become lodged tight!
The case has to be able to expand enough to release the bullet without increases in pressure, any fouling that wedges the casing restricting the bullets release...

What is worse is the wear on the edges of the end of chamber cuts and the bullet can almost weld itself as it is scrapped off by that edge. It doesn't happen often but when it does, it a pain in the ass.

I hope you have an understanding about this but in a pinch and no other ammo is available it can be a viable alternative to get you out of a jam! I have fired the 40S&W from some of my guns and my doing so has provided me the insight that there are risk to doing so!

Usually a barrel is very cheap insurance compared to the cost of an "issue".

Shadow, now that was eloquent!

I had just rote that explanation on Facebook's 10mm group...

thanks for the advise and I will heed it! I hadn't thought about the extractor holding the case. I assumed that the .40 just went deeper into the chamber and was headspacing "properly". that being said, I did go out earlier today and shoot a bunch more out of the 10mm bbl. but they will be the last unless it is a real emergency.

Are the .40 barrels drop ins or do the need fitting?

mine was drop in. EAA says they might need fitting but I think that is just a CYA statement.

My 38 Super barrel did NOT drop in.

I always wandered about shooting .40 in a 10mm. I just never had the guts to try it. I once had a kb when I got distracted and grabbed the wrong powder. I'm a lot more safety conscience now because of it and the thought of shooting .40 in a ten always gave me the heebie jeebies. It is good to know that you can use .40 in an extreme situation. Until that day comes ill just stick to shooting 10mm in my G20. Every day I look at this site I learn something new. You guys are great! Now I am motivated to load some new rounds. I think I will try out longshot when I find some. Any load suggestions?