[MJB_17]

Some calibers are more susceptible to round set back. .40 cal. being one of the worse, due to the shape of the bullet (
flat nose vs. round nose). In any case, you can make a quick and inexpensive "Go/No Go" gauge very easily. Take a stiff piece of card stock, with the card stock stood on a flat surface, place a new round beside it. Mark where the nose of the bullet is, then cut the card stock, so the bullet tip touches the notched out area. When you check a bullet, if the tip no longer touches the cardboard cut out, you know you have set back. If it still touches, your good to go.

Great info, also add .357 Sig to that category of being setback prone. Both .40 and .357 Sig operate at fairly high pressures for handgun calibers in general and are prone to setback as well. I took a class once where the instructor said something to the effect of "if you chamber a .357 Sig round and don't fire it, throw it away." That's a bit extreme, but I get his point. I like my fingers attached and functional.

 

[MJB_17]

If I plan to expend the mag (and chambered cartridge) every month, then I should be ok right? As part of my drill, I want to shoot 50 rnds of target ammo followed by 10 rnds of SD ammo every two weeks.

At that rate of use I wouldn't be concerned unless it was with a very high pressure cartridge like .357 Sig or a +P+ loading in 9mm. Although .40 is known to also be prone to setback, I don't worry about un and re chambering a round provided within reason. It is a high pressure round, but I don't shoot reloads or hand loads and I have a Glock and an M&P and I know both are tested with proof loads that are MUCH higher than anything likely to be caused by setback.