390 Custom Gas Spring


Senor Technoid,

I had a an unusual experience today………….but forget that and let me get to the reason for my note:

A few months back, I bought a used, but like new, 390 from a friend. After a few hundred rounds, the bolt link broke. Having read virtually every word you’ve written, I contacted Rich Cole and ordered two bolt links……….one for the gun and one for my peace of mind.

Low and behold, a few hundred rounds later, the second one broke; which confirmed my paranoia and good judgement in having bought two. I replaced the second broken one and today, after several hundred rounds, it is still in one piece.

However, I called Mr. Cole to order yet another spare today and was asked whether I had been having a problem with breakage. To which I replied in the affirmative and retold my tale of woes. At which point, the person on the phone said that they have had a lot of problems with this part breaking and have discovered the cause………a gas valve spring which is too strong thus causing excessive bolt velocity.

Whereupon he informed me that they are now selling a replacement spring that has been modified to be weaker and thereby allow gas to escape sooner slowing the speed of the bolt. Seemed to make sense so I ordered one.

Couple of questions:

1. What in general do you make of all this?

2. Has excessive bolt speed damaged my gun (Following your advice, I did replace the return spring when I first purchased the gun)

3. Why and how does a problem of this type slip by Beretta?

4. Why do some people who shoot 390s seem not to experience this problem?

On another topic: My gun is a 28″, basically unmodified except for a Frank Glenn speed button which replaces the god awful factory button and makes the gun a pleasure to operate. You should consider getting one. If you want his number, let me know.

I am considering getting one of Briley’s barrel weights that screw on in place of the mag. cap. to add a bit more weight up front thinking that it may smooth out my swing and possibly stop the occasional slowing or stopping of the gun, which I don’t seem to have a problem with during practice, but sometimes experience under tournament stress.

I know the added weight is all in one place, (see, I have read everything you’ve written), but think that in fact having the extra weight near my leading hand might in fact be a good idea.

Also, the weight itself is hollow and one can add as much or as little lead shot as desired. It also looks cool and is easier to take on and off than the damn barrel cap.

Any thoughts you may wish to share would be appreciated.

Thanks
N

Dear N,

First of all though, let me commend you on your intelligence in buying two links. The standard rule of gas guns is to always order two parts to replace one which has just broken. If it broke once, it may again. Unlike O/Us, parts are cheap and easy to install. If I were you , I would also lay in a couple of sets of hammer struts too. Links, hammer struts and some extra springs should be all you need in inventory.

Here’s the situation with that little gas valve spring (the one at the front of the forend with the gas seal washer attached- for you readers who might not have fooled with it). The idea behind this spring and also the clip-shaped one that performs the identical function in the Remington 11-87 is to increase the range of shell pressures that the gun can handle.

When a cartridge goes off in a 303, 1100 or previous gas gun without a secondary gas bleed valve, some gas pressure bleeds through the gas ports in the barrel and pushes against the gas piston driving the bolt assembly rearward. Shells with too little gas pressure don’t push hard enough to cycle the action. Shells with too much gas pressure push so hard that the action speed increases too much and parts begin to break. Shells in the middle work just right.

The idea behind these secondary gas bleeds that the 390 and 11-87 have is that the primary ports in the barrel can be quite large so as to accommodate the lightest load. The large ports permit a great deal of the low pressure shell’s gas to operate the action, so the gun will get enough gas to cycle with that light load. The gas pressure that does come through is NOT enough to force the secondary gas bleed open, so 100% of that light pressure goes to work the action.

With heavy loads, the secondary gas valve comes into play. The theory is that excessive gas pressures come through the primary gas ports like always, but these pressures are high enough to collapse the secondary gas bleed spring (this is the part that Rich is replacing) and bleed off the excess gas before the secondary valve snaps shut and the remaining gas, now at a lower level, operates the gun in the normal manner. Gas pressure is reduced to safe operating levels and the gun can cycle at the normal speed, which saves parts from breaking. It’s a great idea when it works.

Obviously, there are two areas that are critical to the success of this system: 1) the primary gas ports drilled in the barrel, and: 2) the strength threshold of the secondary valve spring.

You would think that any company could drill consistently sized gas holes and manufacture springs of consistent strength. Not so. You would be amazed at what production variances are, especially when you start to outsource parts. Beretta primary gas port sizes varied all over the place in their older guns. It is a tribute that the gun that they worked at all.

Same with the secondary valve springs. This may well be an outsourced part (maybe not) and the resistance levels of all the springs have not been consistent. If the spring is too heavy, as perhaps yours was, the secondary gas valve will not open soon enough. This keeps too much gas in the action and accelerates the speed of the bolt beyond design parameters. Excessive bolt speed accelerates parts breakage. The weakest link in the chain is the “link” that you were breaking.

What Rich has done is to come up with two carefully calibrated secondary gas valve springs. One is a light spring that collapses fairly easily for continuous use with heavy loads. The lighter spring ensures that the valve will open and vent the excessive gas of the heavy shells, thus saving your action.

The other spring is a bit heavier. This assures that the secondary valve will NOT open when using light loads, thus ensuring that all of the light load’s gas goes to operate the action reliably.

You would think that one spring, perfectly in the middle, would do both, but shot shells differ so tremendously in their pressures and pressure durations that this is just wishful thinking.

Most people don’t really have a problem with the gas valve spring because

1) their particular spring came from a lot that happens to be dead on for the shell that they shoot, and

2) they stay with a shell of a certain pressure curve more often than not.

How do you tell if your springs are the right strength for the shell you are using? Simple. Assuming that your mainspring is fresh and that the gun works normally, fire a few of the shells you like to use and note how far the ejected hull is thrown. You want the SHORTEST ejection distance that functions the gun reliably. Five or six feet is about right. 15 to 20 feet mean that the excessive gas has driven the bolt back too fast. This throws the ejects a long way and beats up the action. If the gun throws the hull too far, a lighter secondary valve spring will cure it by opening earlier and venting off the excess gas.

That’s it in a (long and windy) nutshell.

As to screw on barrel weights for the 390- have at it. They are cheap and fun to play with if you are so inclined. While you are at it, don’t forget that the all steel forend nut from the Beretta A-390 weighs a couple of ounces more than the identical looking aluminum one used on the AL-390. It might be all the extra weight that you need. Personally, if I owned a 28″ 390 and wanted a little more stability and follow through, I would try a 30″ barrel. That way you might get the balance you want and also a longer sighting plane.

Boots off. Beer open.

Best regards,

Bruce Buck
Shotgun Report’s Technoid
(Often in error, never in doubt.)

Advertisements
This entry was posted in Shotguns. Bookmark the permalink.

2 Responses to 390 Custom Gas Spring

  1. TH Milstead says:

    Have AL 390 20 gauge 26″ bbl, gun was too light, L means light. Bought 28″ bbl, still too light. Took same to Briley, had them machine plug weighing 4 oz & replaced plastic plug. Problem solved. Better aesthetics.

    Like

  2. Ed Sybert says:

    Cole also sells a set of 6 Beretta 390 gas vent springs, color coded to identify the nominal pounds of pressure needed to compress each spring. The user can select the spring that produces a 6′ – 8′ hull ejection distance. I’ve used my set for about 5-6 years and have had no link breakage or other problems. The set of 6 allows you to “fine tune” your particular shotgun/barrel/load. Happily, my back-up 390 likes the next stronger spring (both mainsprings are replaced at about 10K rounds). -Ed

    Like

Leave a Comment

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.