Slide to Frame Fit
Tightening the clearances between the slide and frame are done with different techniques, depending on the individual situation encountered with each gun. Pistols that come as complete functioning firearms generally have a slide and frame that readily fit together with clearance left over. Slides and frames purchased as parts often are machined to dimensions that result in the frame and slide being unable to fit together due to interference. Depending on which circumstance exists, different techniques are required to make the slide and frame fit together with the smallest feasible clearance.

Interference Removal Fits
In the simplest terms, all that is needed to fit up this situation is to remove controlled amounts of material that is preventing the two parts from fitting up. This requires taking accurate measurements and making a plan to remove the interfering material in an accurate manner. Removal is usually done with a precision surface grinder and/or an accurate milling machine. After the parts are machined to a point that they will just fit together with force, they are lapped with ultra fine compound to a free-running fit, then cleaned and oiled. I charge $85.00 for a machined fit of slide to frame (#301a).

Clearance Removal Fits
To close up existing clearances to make a tight fit, several methods are available to remove the existing clearance. The method (or combination of methods) used is dictated depending on where the clearance exists and how much clearance exists. The methods used consist of peening the frame rails down to remove vertical clearance, squeezing the bottom of the slide to remove horizontal clearance as well as welding and remachining the frame rails to accomplish both. Also, Accu-Rails can be installed to remove both vertical and horizontal clearance. The decision of which is the most appropriate is made after accurate measurements are taken to determine where the clearance exists and how much needs to be removed.
If a particular slide and frame have relatively small clearances in one or both planes, it is possible to peen the frame rails down to remove the vertical clearance while at the same time removing the horizontal by displacing some of the peened metal to the sides. There is a limit as to how much side displacement can be practically achieved with peening. If a gun had for example, .004" of vertical clearance and .014" of horizontal clearance, the slide would need to be squeezed in addition to the rails being peened.
There is a risk involved with squeezing the slide to tighten clearances. Occasionally, a slide will crack while being squeezed. I take every precaution to avoid this happening and so far, I have not had one to crack. I have seen it happen, though and know that sooner or later, it will happen. Should I crack one, I will make it right. The one problem that is created is that if your slide cracks, I may not be able to find a properly roll marked replacement slide. I will make every effort to replace it with an identical slide, but some of these guns have been out of production for a long time and new stock is long gone. I mention this not to alarm, but to inform. I charge $115.00 for a peen / squeeze tightening job (#301b).
If, after measuring, it appears that a significant amount of clearance needs to be removed, welding the rails and remachining may be the best option. It might also be a good option if you have a slide that you consider irreplaceable and don't want to run the slight risk of cracking by squeezing. This is done exactly like it sounds, a controlled weld bead is applied with a TIG welder to add material to the frame rails. After welding, the rails are re-machined and lapped as described above discussing an interference removal fit. I charge $175.00 for a weld and re-machine tightening job (#301c).
Acc-u-Rails are a unique way of solving the slide-to-frame fitting dilemma. They are hardened round steel rods that are installed into grooves that are machined into both the slide and frame. The rods have a very short end section that is bent to 90 degrees and inserted into holes drilled into the frame. This bend retains the new rod in position, yet allows it to be replaced in the future to take up for any minor wear. Acc-u-Rails really shine in two respects: If you have a lot of clearance to get remove and don't want to weld the frame, Acc-u-Rails are your best alternative. The other situation would be if you have a stainless or aluminum frame, installing Acc-u-Rails will give you a tightly fitted, steel-on-steel bearing surface with the slide. This would fix the issue of wear on an alloy frame or galling on a stainless frame. Acc-u-Rails are the most expensive way of tightening the slide to frame fit at $250.00 (#301d).

After inspection, careful measurement and evaluation of your individual needs, I can recommend the best method to utilize. If you are filling out a work order and want a tight slide to frame fit, check off "301a" if it's a pistol being built from the frame-up with oversize rails or "301b" if it's a complete functioning base gun. If it needs "301c" or "301d", I can change it and advise you after I inspect the pistol.