I saw this recently mentioned in a different thread, and the guy who apparently concocted this mod, @Shooter71 , told me how to do it--so I can't take any credit but thought I would pass this on for anyone interested. I believe he does this professionally so if interested I would hit him up.
It was not particularly difficult with the shop tools I had on hand. I have not tested it extensively with various magazines, so if anyone does and can add to this, please do.
Although I had to shorten the AICS mag I had, I was told this mod will fit factory AI and Accurate mags, and I assume also MDT or other metal mags, although I have not tested that. I like my tikka mags when I use this rifle for other stuff, so I was mainly interested in this mod in order to 1) have access to less-expensive and more readily available AICS .308/6.5 10-round mags for spares and overall versatility, and 2) utilize a flush, 3-round magazine for hunting in states that limit capacity, and since I have no need for any more than 3 or 4 rounds when hunting.
The way I have it described here utilizes the MDT 3-round polymer AICS mags in the CTR factory bottom metal. The initial mod is only to the bottom metal so it accepts BOTH aics mags and tikka mags. In this case, I also had to modify the polymer magazine, as it was slightly too long for the tikka mag well.
Modifying the mag was simple--the magazine was .034" too long to fit inside the mag well. I simply put it on my table sander and ground the front end of the magazine down. I measured and taped it off first, and I used a 7/64" drill bit underneath the front to maintain even contact with the sander since the bottom isn't perpendicular to the front of the magazine, and I used a .080" drill bit taped to the fence on my sander on the side to account for that taper--if you try this you'll see what I mean. I'm sure you could eyeball it, I'm just like that. I did not have to modify the feed lips at all on the one mag I had. I'm going to buy a second now that I've tried this and will report back if it requires any other mods.
Modifying the magwell was also simple. It involves 2 mods--filing or milling a shallow tapered ramp into the rear wall of the magwell to accommodate the protruding catch on the back of the AICS magazines without crushing or eroding it with use; and filing off a small lip on the inside of the magazine-retention lever that allows it to depress a bit more.
*The mag well cut, I put the metal in a vise and used a 1/4" square mill file. I measured the height and depth of the protrusion on the magazine and marked those points on the metal, and filed a ramp a bit past the mark, and used the same width as the existing recess for the magazine retention lever.
*A small punch pushes out a pin holding in the retention lever, it is a one-piece part with a coil spring. I was afraid the spring would fly across the room, but it's not under much tension. It was very simple to grind off the lip on the rear, top surface of this catch (you cannot see this lip without removing the lever, but it's obvious once you do), allowing it to retract into it's recess a bit further, providing the magazine with more room as it enters and drops free of the mag well. It's easy to put back into position and tap the pin back into place.
The result is it works exactly as it did from the factory with the tikka ctr mags, but now it also accommodates the positive protruding catch of the AICS mags as well. The 3-round mag I have seems to feed reliably and smoothly from what little I've used it since doing this mod. Looking around on the interweb there appears to be considerable variation in aics-pattern mags, so I'm sure some will work better than others. This MDT one was fairly heavily tapered on the sides--width of the front of the mag was only .827" versus the factory tikka mags at .985. So far this has not affected feeding, and the front-to-back length being a snug fit forces it to stay centered, but it may allow some grit and stuff in there at times. the 3-round polymer mag does not drop free of the gun on its own when empty, but its easy to remove--it's quite light (1.25oz empty) so I have no doubt a heavier or loaded mag will drop free without assistance.
For those interested in such things, my rifle in 6.5cm fed with no issues using cartridges with overall length between 2.67" and 2.805"--there's a little but not much more room than this.
Some pics below to hopefully help visualize.
pic1: the mdt 3-round mag with the front ground shorter
pic2: the ramp partially filed into the magwell
pic3: the mag retention lever with the "lip" partially filed off
pic4: the gap around the bottom of the magazine--note the magwell is beveled both on the sides and especially at the front, so most of this gap that's visible in the photo is also present with the factory mags, but due to the taper on the side of this magazine, especially near the front there is a .080" gap on each side tapering back to 0 at the rear compared to my tikka mags.
pic5: image showing the flush fit
It was not particularly difficult with the shop tools I had on hand. I have not tested it extensively with various magazines, so if anyone does and can add to this, please do.
Although I had to shorten the AICS mag I had, I was told this mod will fit factory AI and Accurate mags, and I assume also MDT or other metal mags, although I have not tested that. I like my tikka mags when I use this rifle for other stuff, so I was mainly interested in this mod in order to 1) have access to less-expensive and more readily available AICS .308/6.5 10-round mags for spares and overall versatility, and 2) utilize a flush, 3-round magazine for hunting in states that limit capacity, and since I have no need for any more than 3 or 4 rounds when hunting.
The way I have it described here utilizes the MDT 3-round polymer AICS mags in the CTR factory bottom metal. The initial mod is only to the bottom metal so it accepts BOTH aics mags and tikka mags. In this case, I also had to modify the polymer magazine, as it was slightly too long for the tikka mag well.
Modifying the mag was simple--the magazine was .034" too long to fit inside the mag well. I simply put it on my table sander and ground the front end of the magazine down. I measured and taped it off first, and I used a 7/64" drill bit underneath the front to maintain even contact with the sander since the bottom isn't perpendicular to the front of the magazine, and I used a .080" drill bit taped to the fence on my sander on the side to account for that taper--if you try this you'll see what I mean. I'm sure you could eyeball it, I'm just like that. I did not have to modify the feed lips at all on the one mag I had. I'm going to buy a second now that I've tried this and will report back if it requires any other mods.
Modifying the magwell was also simple. It involves 2 mods--filing or milling a shallow tapered ramp into the rear wall of the magwell to accommodate the protruding catch on the back of the AICS magazines without crushing or eroding it with use; and filing off a small lip on the inside of the magazine-retention lever that allows it to depress a bit more.
*The mag well cut, I put the metal in a vise and used a 1/4" square mill file. I measured the height and depth of the protrusion on the magazine and marked those points on the metal, and filed a ramp a bit past the mark, and used the same width as the existing recess for the magazine retention lever.
*A small punch pushes out a pin holding in the retention lever, it is a one-piece part with a coil spring. I was afraid the spring would fly across the room, but it's not under much tension. It was very simple to grind off the lip on the rear, top surface of this catch (you cannot see this lip without removing the lever, but it's obvious once you do), allowing it to retract into it's recess a bit further, providing the magazine with more room as it enters and drops free of the mag well. It's easy to put back into position and tap the pin back into place.
The result is it works exactly as it did from the factory with the tikka ctr mags, but now it also accommodates the positive protruding catch of the AICS mags as well. The 3-round mag I have seems to feed reliably and smoothly from what little I've used it since doing this mod. Looking around on the interweb there appears to be considerable variation in aics-pattern mags, so I'm sure some will work better than others. This MDT one was fairly heavily tapered on the sides--width of the front of the mag was only .827" versus the factory tikka mags at .985. So far this has not affected feeding, and the front-to-back length being a snug fit forces it to stay centered, but it may allow some grit and stuff in there at times. the 3-round polymer mag does not drop free of the gun on its own when empty, but its easy to remove--it's quite light (1.25oz empty) so I have no doubt a heavier or loaded mag will drop free without assistance.
For those interested in such things, my rifle in 6.5cm fed with no issues using cartridges with overall length between 2.67" and 2.805"--there's a little but not much more room than this.
Some pics below to hopefully help visualize.
pic1: the mdt 3-round mag with the front ground shorter
pic2: the ramp partially filed into the magwell
pic3: the mag retention lever with the "lip" partially filed off
pic4: the gap around the bottom of the magazine--note the magwell is beveled both on the sides and especially at the front, so most of this gap that's visible in the photo is also present with the factory mags, but due to the taper on the side of this magazine, especially near the front there is a .080" gap on each side tapering back to 0 at the rear compared to my tikka mags.
pic5: image showing the flush fit
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