November 28, 2012

FNP-9 Recoil Spring Solution

Matching the proper weight recoil spring with your ammunition can be a key element to producing consistent slide movement

In a recent test of the $720 FNH USA FNP-9 DAO (double action only) pistol our Gun Reporters noticed that its slide movement was at times clumsy. That is, the slide movement sometimes seemed to stall. The test staff wasn’t sure if the slide was moving all the way to the rear and stopping for a moment or if it was hesitating short of its full cycle.

A number of malfunctions occurred wherein the pistol left a fresh round not fully chambered. Magazines were switched, extractor tension checked, and the gun was cleaned and lubricated.

The problem was most noticeable when firing 147 grain subsonic rounds. Changing to a variety of standard pressure rounds reduced the amount of malfunctions but the gun still felt like it was laboring. When 125 grain +P, (high pressure, high velocity), rounds were loaded cycling was assured but movement still felt uneven and choppy. Both heavy recoiling ammunition and soft shooting loads seemed to put slide movement out of synch.

Would the difference in finish on the silver replacement part offer greater lubricity and allow the spring to work more freely? There was no question that the replacement recoil unit was different from the original. But, was the difference enough to improve performance in terms of feel or reliability?

Matching the proper weight recoil spring with your ammunition can be a key element to producing consistent slide movement and smooth reliable cycling. With little else to go on it was fair to assume that the recoil spring installed in this FNP-9 was too heavy. Perhaps a recoil unit meant for the 40 S&W FNP-40 had been installed in the 9mm test gun. I decided to investigate.

Underneath the slide of the FNP-9 was a recoil system consisting of a guide rod surrounded by a flat wire spring. The spring was captured on the guide rod. Removing the top end of the pistol did not require the operator to work hard at compressing the spring in order to install the unit or take it out. This also reduced the number of parts to keep track of during field stripping and of course prevented the spring from flying off the guide rod to parts unknown. (Parts unknown, you may recall, are never located in the middle of a well lit floor.)

Obviously this design is an advantage for the operator in the field but it does make it difficult to fine tune the gun. Aftermarket parts that offer an alternative to the captured guide rod for the FNP-9 are not yet available. I decided to order a fresh 9mm recoil unit directly from FNH-USA and see if it was any different from the original equipment.

The cost was about $27 complete. This was a nominal cost and besides, I had already ordered a 40 caliber model FNP-40 for my own use. Assuming the 9mm unit was much lighter sprung I would be able to use it in my FNP-40 any time I wanted for practice with less powerful hand loaded ammunition.

The first thing I did upon receipt of my 9mm guide rod and spring was to compare it physically to the assembly originally installed in the FNP-9. Immediately apparent was the color. In each case the guide rod was comprised of two pieces. The end piece that contacted the barrel lug was black.

The length of the rod that seated below the muzzle was silver on the replacement part but black on the original assembly. Checking with FNH USA both units were fashioned from an unspecified alloy. But, the end pieces appeared to be made from a polymer type material.

Thanks to a nick in the surface of the black guide rod I found out that it was silver underneath a coating of black. Each unit utilized a flat wire spring. Squeezing the filament with a dial caliper showed that the wire on both springs was about. 0.016 inches thick. Counting the amount of coils I found that the replacement unit featured one less coil. This would allow it to present slightly less resistance to being compressed.

Would the difference in finish on the silver replacement part offer greater lubricity and allow the spring to work more freely? There was no question that the replacement recoil unit was different from the original. But, was the difference enough to improve performance in terms of feel or reliability?

At the range I tried shooting the FNP-9 with the same ammunition as before. No malfunctions were encountered with the new recoil assembly in place. However, no malfunctions were encountered with the original recoil assembly as well. Certainly there could be other factors in play. Most likely of which was that the gun was now broken in.

But, the cycling of the pistol seemed more consistent and smooth with the silver replacement assembly in place. Several attempts to quantify the difference in resistance were made. A Chatillon trigger pull gauge was used to measure the force of the closing slide. Another measurement was made in an attempt to determine the amount of weight necessary to pull back the slide to the point at which the stop could be engaged.

If we were to agree for the sake of argument that these tests were scientifically accurate then we would have to accept findings that told us there was little difference in performance with either recoil assembly in place. Had the FNP-9 test pistol been shipped with a recoil assembly designed for heavier ammunition i.e. a model FNP-40 pistol? I would say no. I feel that the differences between the two parts most likely represent a change meant to address a problem that had become apparent to the manufacturer.

For peace of mind I chose to continue shooting with the replacement part fit with one less coil to its spring and the “fancy” silver guide rod.