Witness Holes

[born-in-45]

I am wanting to add a couple witness holes to my STI and SVI mags (140 and 170). How do you determine where to drill the holes. Does someone have patterns or measurements available?

[yoshidaex]

it'll depend on the follower/spring/basepad combination among other things.

the mag tuning gurus could offer a bit more insight.

[born-in-45]

Yep, I imagine that would have an impact on location. I'm running Grams internals and DP base pads on all but some are +1, some are +2, and couple are SNL. Besides 38SC and 9MM, I am also looking at doing some 140's in .40

[AlamoShooter]

Drill them one size smaller than you want to end up with. That way when you file off the sharp edges the holes wont be -Too Big

[Aircooled6racer]

Hello: It depends on the feed lips and the angle of the tube etc. The followers don't come into play or the base pads. You will measure from the first round down and so much over from the sides. I did some for the 126mm STI mags a while back and started with a 1/16" hole first so I could pull it over if I had to. I did those so I could see when they were loaded with 10 rounds for IDPA. Thanks, Eric

[kgunz11]

I think it a 11/32 bit that I use, would have to check, it's about the size of the primer. Better use kobalt bits (not the name brand, the coating). Ti bits will be gone after 2 mags. STI mags are 410 Stainless Steel and hard as a woodpeckers lips.

[Black Gun]

Hello: It depends on the feed lips and the angle of the tube etc. The followers don't come into play or the base pads. You will measure from the first round down and so much over from the sides. I did some for the 126mm STI mags a while back and started with a 1/16" hole first so I could pull it over if I had to. I did those so I could see when they were loaded with 10 rounds for IDPA. Thanks, Eric

I am wanting to do some 126mm STI mags for IDPA do you have "approximate" start dimensions ?

Thanks Donald

[kgunz11]

Just load 10 rounds and measure it.

[bpowe]

I haven't done any mag drilling yet but it seems to me the followers will have some input as they determine how the rounds stack in the mag (and thus which side of the mag the hole goes on).

[yoshidaex]

I would think followers play a big part in where round count holes should go. The last time I checked, grams, bolen, Dawson and stock Svi and Sti followers all measured different.

I guess you could get an average of where all followers would end up with x amount of rounds in and have a hole that's not perfectly lined up.

[kgunz11]

Hello: It depends on the feed lips and the angle of the tube etc. The followers don't come into play or the base pads. You will measure from the first round down and so much over from the sides. I did some for the 126mm STI mags a while back and started with a 1/16" hole first so I could pull it over if I had to. I did those so I could see when they were loaded with 10 rounds for IDPA. Thanks, Eric

That's not completely true. The follower depicts which side of the magazine the rounds are pitched to. In an SVI magazine using a SV spring and follower (slightly modified) the 10th and 18th round is pitched to the opposite side of the mag. The geometry of the top of the SVI follower is different than the Grams or Bolen offerings. Here are photos of an example. This is an SVI tube with Dawson +1SNL base pad and SV spring and follower. Notice the witness holes are cut on the right side instead of the left side. This mag has not been polished yet which is evident by the horrible deep scratches left from removing the weld.

[kgunz11]

BTW, a Dawson follower will show 9 and 17 if you drill it like you normally would for 10 and 18. Followers can have an effect, but Grams and Bolens are interchangeable.

[Aircooled6racer]

Hello: I have been using the STI followers for the 126mm IDPA mags in STI tubes. I will try some Dawsons and some Grams tonight some time to see how that works out. Oh just remembered all my mags are packed for the move. It will be a couple of weeks before I can get at them. I will then measure down from the top round to give you the measurement. Thanks, Eric

[HSMITH]

Poking holes without blueprinting the tube is a great way to get a halfass product. If the tubes are consistent you can use measurements or jigs, but taking a handfull of tubes and poking holes in them without them being consistent is going to be ghetto.

[kgunz11]

I agree Howard, I confirm measurements on every tube. I want the primer to be in the hole like those in the mag above.

[Aircooled6racer]

Hello: If the holes don't line up with the ammo inside what would be the point? The other problem with the holes is that they allow more Texas dirt in Thanks, Eric

[Jim Watson]

I wanted witness holes at the 10 and 11 round mark in 12 round STI .45 magazines for IDPA.

I measured. The least tedious method was to take off the floorplate, let the spring protrude against a tabletop to apply light tension to the follower. Then load the magazine with 10 and carefully invert it. Remove the spring and follower and measure to the bottom round. Then mark on the outside, figure to the middle of the primer, and drill, then deburr. There was more difference magazine to magazine than I expected, but it is clear whether there are 11, 10, or fewer rounds loaded.

[Black Gun]

I wanted witness holes at the 10 and 11 round mark in 12 round STI .45 magazines for IDPA.

I measured. The least tedious method was to take off the floorplate, let the spring protrude against a tabletop to apply light tension to the follower. Then load the magazine with 10 and carefully invert it. Remove the spring and follower and measure to the bottom round. Then mark on the outside, figure to the middle of the primer, and drill, then deburr. There was more difference magazine to magazine than I expected, but it is clear whether there are 11, 10, or fewer rounds loaded.

Thanks for the info ! this is how you "just load ten rounds and measure it"

I thought if I could see the rounds to measure I wouldn't need the holes