born-in-45 Posted May 12, 2009 Share Posted May 12, 2009 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? Link to comment Share on other sites More sharing options...
yoshidaex Posted May 13, 2009 Share Posted May 13, 2009 it'll depend on the follower/spring/basepad combination among other things. the mag tuning gurus could offer a bit more insight. Link to comment Share on other sites More sharing options...
born-in-45 Posted May 13, 2009 Author Share Posted May 13, 2009 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 Link to comment Share on other sites More sharing options...
AlamoShooter Posted May 13, 2009 Share Posted May 13, 2009 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 Link to comment Share on other sites More sharing options...
Aircooled6racer Posted May 13, 2009 Share Posted May 13, 2009 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 Link to comment Share on other sites More sharing options...
kgunz11 Posted May 13, 2009 Share Posted May 13, 2009 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. Link to comment Share on other sites More sharing options...
Black Gun Posted May 13, 2009 Share Posted May 13, 2009 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 Link to comment Share on other sites More sharing options...
kgunz11 Posted May 13, 2009 Share Posted May 13, 2009 Just load 10 rounds and measure it. Link to comment Share on other sites More sharing options...
bpowe Posted May 13, 2009 Share Posted May 13, 2009 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). Link to comment Share on other sites More sharing options...
yoshidaex Posted May 13, 2009 Share Posted May 13, 2009 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. Link to comment Share on other sites More sharing options...
kgunz11 Posted May 13, 2009 Share Posted May 13, 2009 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. Link to comment Share on other sites More sharing options...
kgunz11 Posted May 13, 2009 Share Posted May 13, 2009 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. Link to comment Share on other sites More sharing options...
Aircooled6racer Posted May 14, 2009 Share Posted May 14, 2009 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 Link to comment Share on other sites More sharing options...
HSMITH Posted May 14, 2009 Share Posted May 14, 2009 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. Link to comment Share on other sites More sharing options...
kgunz11 Posted May 14, 2009 Share Posted May 14, 2009 I agree Howard, I confirm measurements on every tube. I want the primer to be in the hole like those in the mag above. Link to comment Share on other sites More sharing options...
Aircooled6racer Posted May 14, 2009 Share Posted May 14, 2009 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 Link to comment Share on other sites More sharing options...
Jim Watson Posted May 14, 2009 Share Posted May 14, 2009 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. Link to comment Share on other sites More sharing options...
Black Gun Posted May 18, 2009 Share Posted May 18, 2009 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 Link to comment Share on other sites More sharing options...
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