Colt Improved Sights
D, E, and I Model Forcing Cone Data
Mouth Mouth
Model Calibre Min. Max.
D .22 LR .238" .243"
D .32 NP .330" .335"
D .38 Spl. .370" .383"
E .38 Spl. .363" .378"
E .22 LR .238" .243"
E .22 Mag. .238" .243"
I .38/.357 .370" .376"
Colt's D, E, and I model forcing cones are factory cut on a .160" taper per inch. This works out to about 9°. These cones use a 9° included angle forcing cone cutter. However, the matching 9° tapered plug gauge, shown at right, isn't used to check angle, at all, but instead, to check mouth opening diameter. Ideally, when the correct plug gauge is used, forcing cones should gauge midway between the maximum gauge level at 1, and the minimum gauge level, at 2. Mouth diameter should not exceed maximum gauge diameter. As an option after gauging, an 82* cutter can be used to lightly relieve sharp edges at the forcing cone mouth.
Illustration is exaggerated for example.
Forcing cone mouth diameter, at "D", is critical, and must not be greater than, or less than the factory specified minimums and maximums for the model and calibre as listed above. Bullet deformation is kept to minimum in revolvers with correct forcing cones.
Figure 234A- Illustration shows barrel/forcing cone cross section, forcing cone angle and mouth diameter caution. In revolvers, the diameter of the forcing cone mouth opening is more important than the exact forcing cone angle.
Figure 235- Shows a frame with replacement barrel installed.
The rear face was left long for final adjustment of
barrel/cylinder clearance. A facing cutter is shown installed in the barrel. For a correct surface finish, select a cutter a bit larger than barrel face O.D. With hand tooling, use light cutting pressure to prevent scalloping.
Figure 236- Shows cutting a forcing cone using a Forcing cone cutter, bore centering pilot, and guide. Cut slowly, with light drawing pressure to avoid catching a rifling lands and chattering the cut. Remove very little metal with each cutting pass. Check mouth opening frequently with a tapered plug gauge. See figure 237.
Set Barrel/Cyl. Clearance When barrel installation or
re-qualification is complete, and cylinder headspace and endplay are at
specification, barrel-cylinder clearance can then be set. See figure 235.
While a file can be used for this work, some gunsmiths have difficulty keeping the barrel face parallel with the cylinder. For this reason, I suggest using a piloted facing cutter. With careful handling, this tooling will cut the face smoothly and exactly at 90 degrees to bore centreline.
Factory min. clearance is .003" and max. is .008". With re-qualified or new barrels, I suggest a clearance of .005" with the cylinder at near zero endplay. If clearance is set at or below .004", firing residue may necessitate too frequent cleaning. Some match wad- cutter shooters set clearance just at the limit of .008" to allow for bullet lube build-up.
Cut Forcing Cone
A correctly cut forcing cone preserves basic accuracy by preventing
excessive bullet deformation. Rough cut, off centre, and too large or small cones destroy accuracy. Key- holing can be caused by over size mouth openings and/or compounded cone angles.
D/E/I barrels are made with 9° forcing cones. See data on previous page.
With custom barrelling, a steeper 18°
cutter is probably best for jacketed bullets, while an 11° is near optimum for hollow base wadcutters. Forcing cones cut at 9°, 7°, or 5·5° are more gradual, and are cut longer to obtain the correct mouth diameter.
Cut and Gauge Forcing Cone 1. Begin by gauge checking the
existing forcing cone.
2. When angle is unknown, hand spin [dry] brass lapping heads inside the cone, trial and error, until full brass contact identifies the angle.
3. Install the rod, guide, and the correct pilot and cutter.
4. Slowly rotate the cutter,
maintaining a light and very even drawing pressure. Clean and gauge the cone between cutting steps. See figure 237.
5. Repeat cutting/gauging steps until the desired mouth diameter has been reached.
Note: As an accuracy step, since barrels vary somewhat, the mouth of the finished forcing cone can be set so the lower level of the plug gauge rests flush with the barrel face. Thereafter, the mouth can be enlarged, as necessary. But, for general use, the best setting is just at mid-point of the gauge. See figure 237.
Lap Forcing Cone
Polishing forcing cones to a high gloss serves no practical purpose. However, the walls should be lapped just enough to remove cutting ridges. Silicon carbide compounds, available either as a premixed paste or powder, are very workable; the abrasive sticks to the much softer brass lapping head, making it a precision sander. A #320 or #400 polish will reduce the tendency toward lead build- up in the cone, and make cleaning easier. See fig. 238. Note: After lapping, clean the forcing cone and barrel thoroughly with kerosene and a brass brush.
Figure 237- Shows a tapered plug gauge used to check forcing cones. The gauge shown measures mouth size when a 9°
included angle cutter is used. Other cutter angles require separate gauges. The gauge must not drop past the upper level in a finished cone. Mouth opening is ideal when gauge stops midway between gauge levels.
Figure 238- Shows a brass lap installed in the forcing cone after the final gauging and cutting step. The lapping head must match the cutter (i.e., use only a 9 degree lap, after a 9 degree cutter). Brush apply a thin coating of #320 or #400 lapping paste directly to the lap, to prevent the excess from entering the bore.
Figure 239- Shows a barrel crown that is both nicked and dented. Even the smallest nick at the edge of the bore will engrave bullets, allow pressure blow-by and deflection, and affect accuracy to some extent. "By eye", and freehand attempts at re-crowning nearly always make crown problems worse. Use correct tooling.
Figure 240- Shows a mill set-up for installed barrels using a 45 degree, piloted re-crowning cutter. For this work, the barrel is held in a contoured aluminium clamping fixture. The hand cutter with self aligning pilot, shown below, is similar in design. When used care- fully, this tool will produce acceptable results.
Recut Barrel Crown
Accidents, careless handling, and improper transport, etc., can create a wide variety of exterior nicks, dents, and general damage. Barrel crown damage is one of the worst of this category. See fig. 239. Once barrels and frames have been assembled, re-crowning on a lathe is more than a little difficult. For this reason, most tooling available to the armourer is self-piloting and designed to be used after the barrel has been installed.
1. When re-crowning on the mill, first align the barrel and secure it in an aluminium clamping fixture.
Then centre the cutter pilot [usual pilot- bore clearance is .002-.004"] in the bore. This produces a perfect crown and provides room for chip clearance.
2. Lube, and cut the crown slowly, until the dent or nick is just removed. Trim, at least, to the bottom of the rifling grooves.
3. Remove the cutter and install a brass lapping head of the same angle.
4. Brush the lap with a thin coating of #400 silicon carbide lapping paste and lightly polish the crown.
5. Unclamp the revolver. Clean the bore with a brass brush and solvent. All traces of lapping compound must be removed from the bore. Note: When using hand type cutters, point barrel down- ward, clamp barrel between aluminium vice jaws, then lube from the inside. This prevents irregular hand cutting chips from getting between pilot and bore and scratching the interior.
Figure 241- Shows the plug gauge, or range rod check. Before gauging, barrels and cylinders must be clean- and 100% free of lead, copper fouling, or other residue. Pre-check crane alignment with a thimble gauge before making this check. Since this is the basic test for cylinder/barrel misalignment, the crane must be perfectly straight. Plug gauge check as a routine part of barrel work, cylinder replacement, and always with accuracy related problems. Plug gauges detect irregularities, only, and cannot measure or check barrel diameter.
Bore (and lands) diameters can vary somewhat in D, E, and I models. Factory lands specifications at the muzzle for .38 diameter D and I models run between .345" min. and .348" max., and .346" min. and .347"
max. for E models. Since lands diameter determines gauge head size, a variety of range rod heads should be stocked to accurately check barrel-cylinder alignment.
Lands I.D. determines test gauge size in any given revolver. For example:
If the lands measure .348", a service diameter gauge head would measure .0015" less, or .3465". A match diameter gauge would measure .001" less, or .3470".
At the opposite extreme, with the lands at .345", -.0015" would give us a .3435" service gauge head, and minus .001" a .3440" match gauge head, and etc.
It is unrealistic to expect a service duty revolver to pass the match diameter gauge test with the extremely small gauge clearance involved.
With straight cranes, even small things such as a slightly loose (undersize) stem, or flaring around the serial number inside the crane, can hold the crane slightly off centre and cause interference as the plug gauge enters the cylinder. In some instances, these cranes will just allow thimble gauge/latch pin tunnel entry.
A worn cylinder bolt and slightly long bottom hand and/or low ratchet lug may also cause gauge interference. A loose latch pin can add to this condition.
Compression dimpling inside the bore at the barrel shoulder/frame joint, and/or slight flattening from squeezing the barrel in a bench vice, can cause gauge interference inside the bore. Avoid compression dimpling by careful barrel thread and shoulder preparation before installation, and by always holding barrels with contoured blocks. Always pre-test replacement barrels before fitting.
Figure 242- Shows a close view of a factory tuned Python model action. When examining custom factory actions, you will find that fine tuning is basically just a matter of degree. This work involves careful hand fitting to a high level of action smoothness, precision sear and strut work, and tuned, lightened springs and trigger pull for .38 Spl. match and competition use.
Tuning Factors Necessary in Match Actions- 1. Instant bolt pick up and correct bolt drop timing.
2. Correct hand/ratchet fit and "equal" ratchet lug surfaces.
3. Perfect crane/cylinder/barrel alignment.
4. Barrel and cylinder must pass the plug gauge test.
5. Single action sear set for not less than minimum safe pull, without push-off.
6. Strut pick-up surface at top of sear is polished for smooth D.A.; top of hammer toe, and S.A. pick-up surface under sear is polished for smooth S.A.
7. All action drag factors are eliminated. See figure 243.
8. Mainspring tension is correctly adjusted. See figures 173 and 174.
Also, in factory tuned Pythons, cylinder bolt springs are slightly shortened.
Warning: When mainsprings are adjusted to minimum factory trigger pull specification, tension is lowered on both sides of the spring. But, the most obvious effect is that mainspring pressure is less at the trigger. When mainsprings are adjusted to lower tension, or when lighter custom springs are used, the force of firing pin impact is also reduced. Lightened mainsprings and correspondingly light trigger pull make match tuned actions impractical and unsuitable in both field and service duty use for two basic reasons: the possibility of hammer release before the revolver is on target; and, secondly, lower main- spring energy may deliver questionable performance with standard ammunition.
Figure 243- At right, shows possible action drag areas that may require adjustment or polishing to improve overall action smoothness for match or competition use. Colt has a long standing reputation for superb "out of the box" production revolver actions. But, custom tuning takes action smoothness to the point of near perfection.
After final tuning the S.A. sear, D.A. strut, and adjusting mainspring tension, the rest of action tuning is simply refinement, i.e., the careful elimination of factors that cause drag. Drag and friction are always additive; a little friction here, a little drag there, can add up to a fair amount of resistance. Action drag areas, and contributing causes of friction, are listed in the order usually found:
1. Hand- when incorrectly bent, insufficiently edge chamfered, etc., the hand may drag, dig into the frame, or catch the sideplate. When a hand is too thick, or has rough sides, it will bind or drag between frame and sideplate.
2. Hand- when let out too far, the forward point of the top hand may extend past the ratchet recess and drag as the cylinder is opened and closed. The hand may interfere with shell heads when front surface fitting is incomplete.
3. Hand pivot pin- if bent, or damaged by vice jaws, can make the hand sticky.
4. Bottom hand- when long, can hold single action sear position low and cause the hammer to drag or bump the sear on release. Caution: this also can be caused by a long sear extension.
5. Hand's tensioning cam- if mis-cut at a wrong or reverse angle, can drag, back up, bind the rebound, and stick the trigger.
6. Hand's rebound slot- when rough, the surface can drag or bind the side of the rebound lever. This also makes the trigger sticky.
7. Rebound lever- when bent, rough, and/or too wide, the lever can drag the frame, sideplate, and hand- and may also prevent trigger return.
8. Rebound lever- if misfit, the hammer seat may rebound the hammer too far forward, and cause safety drag and/or safety interference.
9. Hammer- may drag the frame or sideplate if too wide, and/or when the sides and skirts have been unevenly polished. When the hammer pin hole is tight, the hammer may drag on the pin.
10. Hammer skirt- inside edge may drag or catch the safety. The hammer skirt may need slight chamfering on the frame side for safety/link pin clearance.
11. Frame- high spots, ridges, and/or burrs may drag the hammer or trigger. A nick or burr at the inside of the sideplate joint may drag the hammer.
12. Frame- a sharp edge or machining ridge at the ratchet recess corner can interfere with the hand and/or ratchet. Safety recess burrs may drag the safety.
13. Safety- high or misfit link pin heads may drag. The safety lever may drag against the trigger. The upper safety may drag inside its frame recess.
14. Latch pin- the back of the latch pin may drag against the upper safety as it cycles.
15. Trigger- when the bearing shoulder is too wide on the left side, trigger will drag between frame and sideplate. Without side clearance, the right side of an I model sear may drag the safety lever. If the trigger pin hole is tight, the trigger may drag on the frame pin.
16. DA strut- a flattened point and/or incorrect strut let-out can produce rough DA release. Also, when the strut is tight or sticky in the hammer body, the strut can slow or stop trigger return.
17. Mainspring- excess tension overloads, and drags, the SA sear point. A sharp bottom spring end may drag the rebound.
A Few Last Words About Action Tuning and Custom Work . . .
Times have changed for the craftsman. These days, if you do action tuning and custom work, you brush shoulders with a bad dream come true called product liability, in much the same way as would a
manufacturer. If you are not already familiar with this subject, it might be worthwhile to talk it over with your attorney. Product liability could be defined as "the modern road to riches." With the help of crafty lawyers, more and more jerks are now retiring early- and on your nickel.
1. Remember that custom gunsmithing is creative, and therefore different than basic mechanical repair work. When you enter this field, you are the designer.
2. Don't do patch work, ever. When you know that a trigger, barrel, cylinder, frame, etc, actually requires replacement- replace it and be done with it.
3. When doing machine work, make all set-ups carefully and on an individual basis. Remember that, with mass produced products, no two are precisely the same. Always recheck the set-up, before cutting.
4. Never alter a safety system. Replace mis-matched safety parts. And, always recheck safety function, since you may be sued for any mishap, thereafter.
5. Do only custom work that you know is safe and properly useful. Turn down odd requests for things such as extremely light trigger pulls, cutaway trigger guards, and etc.
6. Turn down every "opportunity" to perform low quality work. Whatever work you accept, do it well.
However small, every job you do becomes a part of your reputation.
7. Do only the highest quality custom work, and price it accordingly, as quality work. In this way, you will develop a high quality clientele.
8. Be selective, work only for those who can appreciate fine work. Remember that high quality custom gunsmithing goes beyond craftsmanship; somewhere along the line, it becomes an art form.
The very best things have always been made by hand: the finest automobiles, the best shotguns, and the most accurate target revolvers. Well crafted firearms, when properly cared for, will last and can be enjoyed by generations.
Figure 244- Shows a match tuned Colt Python set up in a Ransom Master Series Machine Rest with I frame grip inserts. The optional base shown provides windage adjustment. -Machine rest photo courtesy Ransom International Corp.
Set sights on the vertical centreline of the target and at the 6 o'clock hold position. Minimum test firing distance is 50 feet. The 6x3 test, discussed earlier, uses standard factory, or service ammunition. The 6x6 match accuracy test uses match grade wadcutter loads.
Six rounds are fired through each chamber. The cylinder is marked with a white grease pencil, and that chamber is always fired first. Uniformity (or repeatability) is the rule: ammunition must be from the same lot so that all bullets, cases, powder, primers and shell crimp are the same.
Check all six targets. Points of impact should be well grouped, similar, and with no more variation than would be standardly produced by the ammunition used. Problems having to do with the sights, or a specific chamber or ratchet lug, etc., are easily detected. Also see figures 178 and 241.