Weihrauch HW90 Tuning Guide – How To Do It, What Results To Expect

This Weihrauch HW90 tuning guide has been planned ever since we completed our review of this beautiful air rifle back in November 2022.

The HW90 earned a HAM Gold Award on test. But at that time we didn’t check-out one of its highlight features – the ability to change pressure in the gas ram. Then, Winter weather in up-state New York delayed work.

But now – thanks to HAM Tester Eric Brewer’s hard work – it’s done and we can all share the results of his labor. Thanks Eric!

Oh yes, changing the HW90 gas ram’s pressure does change its shooting performance. A lot! Keep reading to find out how much…

Weihrauch HW90 Tuning Guide – How To Do It

We used a .177 caliber gun because that’s what we had from the test review. But the same principles would apply to any caliber or pellet you want for the HW90.

As Eric will readily confirm, you’ll need to be focused and precise when tuning the HW90. Specifically, he focused in tuning the gun for accuracy with a specific pellet. As Eric’s a Field Target enthusiast, he decided to tune the HW90 for a mid-weight domed pellet. He also decided to shoot targets at 35 Yards, that being a typical FT range.

In fact, he worked through the tuning exercise for two of his favorite pellets – the 8.64 Grain H&N Field Target Trophy and 8.44 Grain JSB Exact – for HAM readers’ benefit.

Having decided on the pellet to be used, there’s a couple of other necessities:

1. Plenty of targets and pellets!

2. A chronograph. (Well you could try without one, but you’d be missing a lot of useful data).

3. The special Weihrauch pressure gauge and pump.

4. A computer with spreadsheet (Excel, Numbers or similar) makes analyzing the results much easier. (I did this – the easy part!)

For this Weihrauch HW90 tuning guide, Eric shot targets for both pellets first at the highest – 20 Bar – pressure in the gas ram. Then he reduced the pressure to 19 Bar and repeated the test. Ditto with 18 Bar and all the way down to 10 Bar, one Bar at a time.

Weihrauch HW90 Tuning Guide

Eric found that the gas ram pressure was reduced by simply pushing the gauge into the port at the rear of the gun. About 8 – to 10 insertions was required to reduce the pressure by 1 Bar, he established.

Very importantly, the screw covering that port MUST be replaced fully home every time after changing the gas ram’s pressure. Don’t forget to do this!

Conforming to HAM standard practice, Eric fired 10-shot groups for every target. The chronograph readings were averaged for the 10 shots each time. He also secured the stock screws with blue Loctite to prevent any inaccuracy developing from that source.

Then he started shooting…

Weihrauch HW90 Tuning Guide – Test Data Analysis

First, let’s establish a benchmark by seeing how Muzzle Velocity changed as the pressure in the gas ram was reduced. Here it is for the Field Target Trophies…

Likewise for the JSBs…

As you can see, both pellets gave similar velocities at any specific pressure in the gas ram. You’d expect this to result in very similar Muzzle Energies, too. The chart below shows that to be the case.

Weihrauch HW90 Tuning Guide

As expected, the cocking effort became easier as the pressure was reduced. Below about 14 Bar in the gas ram, the cocking effort was MUCH less, Eric found. Ditto with the recoil.

Having established that the base FPS data is consistent and solid, now we can move to areas where the difference becomes less predictable.

The charts above show the AVERAGE Muzzle Velocity for each 10-shot string. However, the shot-to-shot variability was far from consistent and predictable – as the next chart shows.

Weihrauch HW90 Tuning Guide

Overall, the average Standard Deviations were quite similar – 5.02 FPS for the JSBs and 6.06 FPS for the H&Ns. Both pellets gave precisely the same lowest Standard Deviation – an outstanding 4.00 FPS – at the 16 Bar setting.

However, the Field Target Trophies really spread out at 10 Bar!

But of course, the purpose of this Weihrauch HW90 tuning guide is really accuracy. To determine this, Eric measured the Vertical and Horizontal CTC values for each chart.

I then multiplied these two values together to represent the area of the group, in square Inches. Yes, I know this is rather an unsophisticated approach. However it was applied consistently and everyone will be able to do it, too.

So here’s how accuracy plaid-out for the JSBs…

Weihrauch HW90 Tuning Guide

The chart immediately makes it clear that the 8.44 Grain JSB Exacts really did not like to shoot in our HW90 at gas ram pressures between 11 and 13 bar. They generally gave best accuracy at higher pressures. Yet there was a great group at 10 Bar. Hey, who knew???

For the 8.64 Grain H&N Field Target Trophies, we see a somewhat different pattern of accuracy. The FTTs obviously gave best accuracy at lower gas ram pressures.

Weihrauch HW90 Tuning Guide


Amalgamating these two charts, we can get a direct accuracy comparison between these two pellets in the HW90 tested by HAM.

Weihrauch HW90 tuning guide

Weihrauch HW90 Tuning Guide – The Final Result

Overall, the best accuracy was produced by the H&N Field Target Trophies at 11 Bar gas ram pressure. The average Muzzle Velocity was an ideal 750 FPS, giving 10.81 Ft/Lbs of Muzzle Energy and a Standard Deviation of 5.00 FPS.

That’s an outstanding 10-shot group at 35 Yards with a breakbarrel. Great shooting, Eric!

Weihrauch HW90 tuning guide

Compare that to the best target – itself not at all bad – that was achieved in the comprehensive HAM test review. That was shot at full gas ram pressure with H&M Baracuda Match pellets at only 25 Yards.

So what about that HW90 pump? Well, you’re going to need that to raise the gas ram pressure back to the level you want after the testing is completed!

This Weihrauch HW90 tuning guide clearly demonstrates the great benefits that can be achieved by some time and experimentation.

Of course, every gun will be just a little different. Also you may use different a different caliber – there’s a choice of .20. .22 and .25 cals – and different pellets. You may want to tune your HW90 for hunting, rather than Field Target and so have different requirements.

But this HAM Guide give you the basic techniques for tuning an HW90, together with ways to analyze the results and what to expect (even if it’s the unexpected). Have fun!!!

Weihrauch HW 90 Air Rifle
HW90 Gauge
HW90 Pump