Rifle Grouping Problems, Perplexing Solution

Every once in awhile we may be faced with a gunsmith/shooting situation that turns out to be more elusive than anticipated. I had built a rifle chambered for the 6x45mm cartridge as one of a quadruple switch-barrel set up. The chosen barrel was a Criterion six groove with a 1:8 twist which would stabilize bullets to just over 100 grains.

I went with a Model 700 Remington short action, originally chambered for .221 Remington Fireball, circa 2008, as my choice in building a super accurate, medium-weight rifle capable of extreme accuracy. The sear at center shows where the firing pin cocking piece engages when ready to fire. Following my customary diagnostic procedure, the action was precision bedded into a Bell and Carlson Medalist varmintweight stock without floor plate, but a magazine cutout did exist.

A bedding pillar/loading ramp was bonded to the magazine cutout as an integral unit replete with central action screw. Sighting equipment left nothing to guess work with a Talley Picatinny mounting system and a well-proven 32X target scope. There were no compromises. From the outset, including the bore break-in, the rifle displayed ordinarily fine accuracy with a number of good bullets and loads. But there was a lingering concern.

Along with the near perfect groups there were some questionable fliers and groups that didn’t quite measure up. As I dry fired the rifle one day, with the bolt handle fully down, the bolt handle The firing pin cocking piece race is clearly pictured here immediately forward of the rear tang screw hole. jumped upward—enough to toss a penny a half inch upward. This left me with the nagging concern that this may be initiating a level of vibration sufficient to cause changes in the otherwise fine accuracy of this rifle.

Anyway, it set the stage for some serious investigation on my part with special emphasis on the bolt and firing mechanism of this rifle. Perhaps over-simplified, the basic cocking and firing of a common bolt action rifle could be described as follows: As the bolt handle is lifted upward, the internal firing pin and cocking piece is withdrawn from its spring-loaded forward position in the bolt through a camming action within the bolt plug, approximately 0.280 inches travel on the Model 700 Remington.

The forward rounded end of the firing pin cocking piece cams into position in a notch at the rear end of the bolt, allowing the cocked bolt to be released and withdrawn from the receiver. As

As the bolt handle is pushed forward and lowered, the firing pin cocking piece engages the sear ready for firing. The flat surfaces of the bolt plug effectively control and limit the rotational function of the bolt and cocking piece. This clearly shows the flat surfaces of the bolt plug and firing pin cocking piece.

The lower bolt required fit and function enhancement referred to in text. the bolt handle is pushed forward and lowered into the receiver, the spring loaded cocking piece is in direct alignment in its slot, directly above and rearward of the trigger sear. During this stage the opened bolt handle is positioned directly above its slot in the receiver.

The rifle is then cocked by a firm push A 0.035" shim placed between bolt handle and mortise proved useful in earlier accuracy evaluation. and clockwise turning thrust of the bolt handle as the firing pin cocking piece engages the trigger sear. The bolt handle can then be partially or fully lowered into position to its normal fire position. Typically, as a rifle bolt is fully closed, as viewed from behind, there should be a perfectly perpendicular engagement and alignment of the cocking piece and sear.

But weird things can take place within the bolt rotational tolerance as the bolt is fully closed, particularly where such tolerances are deemed excessive. I noticed some free play here earlier in the aforementioned bolt, but didn’t pay much attention to it until later during further experimentation. As I viewed the action from directly behind, I raised and lowered the bolt handle a few degrees in each direction.

There was considerable rotary tolerance or movement by the cocking piece and bolt plug surrounding the firing pin assembly. With the bolt handle fully down on an empty chamber, the unrestricted bolt handle would rise a full 3/16th inch from its bottom-most position on firing. With a cocked rifle it could actually cause the cocking piece to separate from the sear through repeated partial raising and lowering of the bolt handle.

Further Searching My next course of action was an attempt to re-create the intermittent poor grouping by manipulating the bolt rotation to varying positions during actual shooting. For this experiment I chose a couple of my well-established loads. Please remember these tests were being performed on a highly accurate rifle, not an ordinary hunting rifle.

I began by shooting a few groups with the bolt handle in the normally closed, but not forced down, position on cartridges that chambered with little felt resistance on the bolt face. This essentially allowed the bolt full freedom against the headspaced cartridge. The first two groups were excellent at around 0.265 MOA average

Both the bolt plug housing and firing pin reflect these changes, which led to a need for corrective action. Shims begin as 0.035" thick stainless steel. using 65 grain Berger bullets and 24 grains of Accurate 2015 powder. I then used the same load to record a 0.495" group that displayed vertical stringing of the shots as the bolt had been manipulated to different positions for each shot for this group.

At the conclusion of these earlier tests the rifle was definitely sending me messages, as guns so often do, and we fail to get the picture. I must point out here that test results such as these could only be concluded with a rifle capable of extreme ac- This is perhaps the last place one may look to as a cause of accuracy defects in a rifle. curacy with groups well under half minute of angle, shot with a perfect benchrest technique.

I finally concluded that full closure of the bolt handle on easily-chambered cartridges were the most likely to cause a bullet to stray from the group. I could readily duplicate this condition, which told me I was on the right track. Analysis Into The Problem As was noted by observing the position of the firing pin head and bolt plug assembly on full closure of the bolt, a malalignment of the parts was obvious.

The right-hand side of the bolt plug (part that surrounds the firing pin assembly) made contact with the receiver while the left-side was raised a full 0.060". Meanwhile the firing pin cocking piece is engaged into the sear, ready for firing— albeit not truly square. Then, on firing, the parts tend to change alignment as the bolt plug rotates and falls back 0.016" toward normal, thereby creating a negative effect.

Such motion creates a moment disturbance within the bolt and receiver sufficient to cause a ripple effect or barrel vibration and ensuing erratic bullet grouping. Manipulation of parts immediately prior to firing clearly bore this out. In my previously published work on barrel and action vibration and dynamic motion I alluded to methods of identifying and controlling various aspects of these insidious processes.

Some of these included the general fit of parts critical to the makeup of a rifle, all the way from barrel fitting to action bedding, which can set up unpredictable motion about the action and barrel affecting bullet departure, hence poor grouping. While we could get lost in this discussion, we could also correctly argue that barrel motion can, indeed, originate in the action of a rifle.

Making Corrective Measures As an experimental corrective measure I first placed a thin metal shim in the slot where the bolt handle engages the receiver cut out. This did not interfere with full closure of the locking lugs as the bolt assembly assumed a more square and vertical position. Bolt handle bounce on dry firing was much reduced in this way. Further, as I chambered a cartridge I raised the bolt a few thousandths of an inch,

The pair of targets on left show a level of accuracy readily acceptable by most before measures taken to further improve it. Both targets on right show measurably improved accuracy following corrective measures. These are just a few of the many representative groups that were used in the analysis and correction of this unusual grouping problem.

Loads shown here were made up with Berger 105 grain Hybrid bullets ahead of 21.5 grains of Norma 203 powder. thereby eliminating or reducing the possibility of rebound bounce on firing and hopefully gaining some fractional improvement in grouping. Groups noticeably improved with virtual elimination of fliers. During this procedure it is important to see that the firing pin head is positioned perpendicular and square within the bolt plug, which aligns the firing pin cocking piece to rest squarely over the sear.

I believe this is extremely vital to uniform lock time—the time interval between sear release and firing pin impact. Variance in lock time can, indeed, affect rifle accuracy where extreme accuracy is being sought. Another point worthy of mention is this rifle is a switch-barrel with the added bolt of .308/.30-06 size. These switch barrels are chambered for the .22 PPC, .257 Ackley Improved, 6x45mm and .221 Remington Fireball.

The also fits the smaller bolt face but wasn’t quite prepared for me to fully evaluate its performance relative to the troublesome bolt. The bolt used for the .257 Improved and the .22 PPC performs remarkably well with good bullets. The two bolts measure out exactly the same from bolt face to the posterior surface of the locking lugs with my bolt face/ locking lug comparator.

Locking lug surfaces are perfectly symmetrical, as are the bolt face surfaces on each bolt—which rules out this aspect as a causative or contributing factor of inaccuracy. The secondary bolt used on this rifle differs also in that it remains perfectly static as the rifle is dry fired with the bolt handle fully down with no rotational movement of the bolt plug and firing pin assembly as the bolt handle is lifted and lowered.

This told me that alleviating bolt plug and cocking piece rotation on the offending bolt could be the logical solution in solving the problem… and so it was. As I more carefully studied the cocked action directly from behind, while lifting and lowering the bolt a few degrees, both the bolt plug shroud and the integral firing pin cocking piece assembly rotated together—but not to the same degree. I immediately saw a solution to of putting the bolt to sleep.

The lower margin of the bolt plug would clearly raise and lower from the bolt locking lug races. A properly fitted shim on each lower margin of the bolt plug should clearly stabilize the excessive rotational effect with a reduction or total elimination in vibration being transmitted from the bolt and action, right through to the barrel. The work was rather straight forward. I simply measured the bilateral spaces between the undersides of the bolt plug and bolt locking lug races in preparation for the shims.

The fitted shims would be bonded to the undersides of the bolt plug on each side. I found some stainless steel of proper thickness, fitted the parts to proper dimensions and bonded them in place with J-B Cold Weld. A short session of fine filing and polishing and the job was done. Sometimes a sort of stop-gap effort or temporary expedient will guide us on our way in problem solving as my earlier efforts did.

I was very pleased to experience more consistent precision performance from the rifle along with a large measure of satisfaction. Conclusion Few aspects surrounding extreme accuracy performance of a rifle are more thought provoking than the effects of barrel motion and the causative factors in dealing with it. Vibrations, however minor, originating in the rifle action/receiver, can, indeed, be transmitted throughout the entire rifle resulting in erratic bullet impact.

While identification and correction of these, sometimes transient, problems may defy our detection for awhile, observant analysis will often pay off. There can be a great deal of satisfaction in discovering the answer… even though this road may be our longest walk. AG