Blog #18 Barrel Droop in air rifle barrels
Barrel Droop has nothing to do with the ageing of your air rifle, because you droop with age it doesn’t mean that your rifle does too. I constantly get guys telling me that the barrel on their air rifle droops because it’s old and due to constant cocking (no pun intended) action while loading. Bottom line guys, air rifle barrels are a lot stronger than that and no matter how many times they are loaded (“Break” barrel models of course…), they will not bend.
What Causes Barrel Droop?
I am going to stick with air rifles here. The “Break” barrel model air rifle has more moving parts and so is subject to wear, especially through lack of lubrication or poor maintenance. The main clevis pin that the barrel hinges on is subject to wear as is the locking detent or latch on some models. However, even new air rifles, “Break” barrels, fixed barrel models like the Weihrauch HW77K and even PCPs can have a percentage of “droop” or misalignment. This is caused by tolerances in mating surfaces and in most cases goes un-noticed.
When we use the term ‘droop’, this can mean misalignment to the left, right or upwards, it does not have to mean that the barrel points down, though in most cases it does with air rifles. In a lot of cases we do not realise that the barrel has some droop or is misaligned to some degree as the telescopic sights tend to compensate for the misalignment issue.
In most cases that I have come across, barrel droop is due to the machined mating orifice in the breach being out of alignment by a very small amount. The very small error in alignment is caused by machining tolerances and is difficult to detect using common lathe tools and so it is often overlooked or undetected. Early Diana air rifles were also commonly attributed to having barrel droop, so it is quite a common fault. Other causes are misaligned bores where the actual bore has been drilled through at a slight angle, though this is fairly rare, I have seen it on several air rifles over the years.
Damaged crowns have also been blamed on barrel droop, so before you go pulling stuff apart, take a look at the barrel crown to make sure there is no “ding” in it. If there is, re-crowning in a relatively simple and cheap fix when done right.
How to check for Barrel Droop?
I’m asked this a lot but can’t help wondering if the issue is not evident then why look for it? So in case you guys think you may have a barrel alignment issue, try this:
- Wind out both the side and top turrets individually to the end of their threads – DO NOT GO NUTS HERE, just apply enough pressure to insure you are at the end of the thread. Any more than that and you may do some damage.
- Then wind the turrets in till they bottom out while counting the turns in doing so.
- Lastly, wind the turrets back to the centre by dividing the total turns on each turret by 2.
- Then shoot the rifle from a rest into a target at say, 20 metres.
- You should be in the black, however, if the group is way off then I suggest that you do the following:
- Swap the scope for another and repeat the test. This will establish whether there is an issue with the barrel or the original scope.
- If the results are the same, that is the group is way off the centre, then do the following:
- If your air rifle is a PCP and the barrel can be rotated, do so by 180 degrees and test fire again.
- If the deviation moves around 180 degrees then have an issue with the barrel not the mating surface. If the shot deviation is random, that is it moves from the original POI, then look at the mating point of the barrel.
- Re-machining the mating joint is an expensive alternative and not always successful. I would opt to fit Droop Mounts or 25MOA scope ring inserts to compensate as this is the quickest and cheapest alternative.
- If you cannot rotate the barrel do a physical inspection of the bore and barrel mounting.
- On fixed barrels like the HW77/97 series Weihrauch air rifles there is little you can do other than a barrel swap and there is a difficulty in doing so. If this is the case, I would fit a Droop Mount, 25MOA spacers or quit the rifle as the expense of rectifying it would be more than its worth with results that cannot be guaranteed.
- There is one other issue that I have come across and that is the shooter him/her self. Having had a HW100T sent to me 2 years ago with the owner complaining of barrel droop, I tested the rifle to find that is was true and correct. So if you reach this point, may I suggest that you get a buddy to shoot the rifle once you have centred the reticle. If he gets the same results as you then the problem is with the rifle or scope, and if his results are different, then the problem is you.
- “Break” barrel air rifles have several moving parts between the barrel and breach that unjustly worries guys as they anticipate a lot of wear and movement in these moving parts. Any droop with a “Break” barrel air rifle is evident when a scope is fitted and not discernable when using steel sights. Why? Because the fore and aft steel sights are mounted on the barrel which is straight whereas the scope is mounted on the breach housing.
Fitting Droop Mounts or 25MOA spacers.
The Droop Mounts are straight forward to fit like any other scope mount. I would however, caution you about using such droop mounts on a heavily recoiling air rifle that may well send the droop mount on its way. My take on this is only use a Droop Mount on a PCP to play it safe and these can be purchased here.
On a spring powered air rifle I prefer to use the 25MOA tapered bushings that go under the scope inside the ring. You will need to check that the arrows point forward and the each bushing is positioned as marked (Front bushing – Rear bushing)
After fitting the bushings carry out a repeat of the above tests and you can alter the MOA to some degree by moving the scope rings in closer together to increase the MOA or move them apart further to decrease the MOA. This is not an exact art as measuring the exact barrel droop in Minutes of Angle is beyond most people.
I have come across numerous scopes that have had small shims made of Coke or Beer cans mounted under the scope and between the ring. A bit untidy but it works much like fitting the 25MOA bushings.
Summary on Scope alignment.
I think I need to point out here that you can’t just go adding shims to the rear mount until you have compensated for the droop. There is a real danger that you will bend the scope due to the differential in heights and alignment between the front and rear scope ring. The misalignment is a ‘stepped’ drop between the rear and front ring with both mating surfaces remaining parallel to the horizontal while the scope tube is at an angle, or as an Aussie would say, “it’s on the piss…”. I have seen this on 3 occasions over the last year so it does happen.
Before you get too involved in rectifying barrel droop in your old air rifle, consider the cost as there is a better than even chance that the gun is a financial write off when taking into account the high costs of parts and labour.
Author: Ian McIntosh June 2016