How much spring?

[RustyBuzz]

This winter, for a bit of a project I’m going to make a reduced diameter, short action 77. The plan is to reduce the cylinder diameter by 3mm and reduce the action length as much as poss while keeping the velocity in .177 to anywhere between 750 - 770 FPS with an 8 grain slug.

So the air chamber will be reduced to 24mm diameter, the stroke length to around 75-80mm.

So my question to all you shed tuning brain boxes is how much spring do you think I will need to squeeze in to achieve the desired velocity? This will determined how much I can take off the action.

All help appreciated,

Richard

[Shed tuner]

This winter, for a bit of a project I’m going to make a reduced diameter, short action 77. The plan is to reduce the cylinder diameter by 3mm and reduce the action length as much as poss while keeping the velocity in .177 to anywhere between 750 - 770 FPS with an 8 grain slug.

So the air chamber will be reduced to 24mm diameter, the stroke length to around 75-80mm.

So my question to all you shed tuning brain boxes is how much spring do you think I will need to squeeze in to achieve the desired velocity? This will determined how much I can take off the action.

All help appreciated,

Richard

When you say reduce by 3mm, the std is 25 or 26 ? so 22 or 23 ?

my hw80 is 24mm, with an 81mm stroke. I have a 22mm and a 23mm TX (~94mm and 88mm strokes respectively)

Now I'm not sure what you mean by "how much spring"... but if you mean action length, you can just make it the same as an 80. Or if you want a spring length / spec preload...?

HOWEVER I really wouldn't reduce the action length.. the longer actions with more spring room just shoot better than shorter action guns. The difference between spring compression force cocked and at rest is just less, resulting in a smoother action. Remember how nice those long cylindered FWBs shot ? But the 24mm will need a weaker spring that a 25 or 26, so you can carbine the front end a bit.


There are other ways you can cheat to get back some spring room if you shorten the action.. e.g. setting the guide back into the end block, and scooping say 5-10mm out for the spring to sit back into (retaining the threads the same) - easier with a slightly smaller spring.

[NickG]

I agree with everything John says. I have a 22 mm diameter 97 running the maximum stroke you can get with standard latch rod set up in non rotating, so 85 mm stroke , it runs a small titan no 12.
On spring room remember longer is better and yo can steal a bit in the piston nose, only need 10 mm thread length .

[TonyL]

Great tips from two of our very best and most adventurous re-engineering bods.

Will look forward to hearing how this project progresses, Richard.

What type of piston seal are you thinking of using?

[RustyBuzz]

Soz chaps, let me clarify.

My intention is to make a smaller, lighter, better handling 77. Ideally around the same size as a Sport. I have no experience with anything under 25mm so was wondering roughly how much room I need to allow for the spring.

Richard

[Shed tuner]

the stroke goes up with reduced dia (reducing spring room), but so does the efficiency, so the short answer is "about the same"...

Why not stick your reduced dia comp tube and piston in a regular 77, and then play with adding solid spring spacers to the rear (and tuning the spring / length), until it becomes unpleasant to shoot / fails to make power. Then measure the spacer you have used, and you know you can reduce the action length by that much. But I'd guess you won't want to reduce it much at all - and if you are only taking off say 20mm, is it worth it ? 40mm I guess you'd notice - but I can't image that being possible in terms of spring room - although ironically it might be (just) with a 26mm bore.

You can do the spacer test with a full size 25 or 26 with a short stroke (nose extension is fine) and see how much you could trim (Add the nose extension length to the spacer to get total dead space you've introduced)... let's say it comes out at 30mm total. Probaly worthwhile in terms of action length reduction. Now you know you will have to add (back) 10mm of stroke for your reduced dia version, so you are down to 20mm dead space. Getting marginal.

Then overlay the "end block scooping" method to get another 5mm or so.

If you want to give me a bell to discuss, just sent us a PM. Now is the season for lockdown projects... I'm about to build a britannia long stroke.

[maximus]

Hi Richard, I am interested in how you intend to shorten, the action, is it from the rear? Are you going to pin the end plug in if so? It would be difficult to add threads in and get it to tighten up correctly.

[Shed tuner]

Hi Richard, I am interested in how you intend to shorten, the action, is it from the rear? Are you going to pin the end plug in if so? It would be difficult to add threads in and get it to tighten up correctly.

the thread pitch is about 1.5 mill, so simply fine tuning the length slightly should take care of indexing.

[maximus]

the thread pitch is about 1.5 mill, so simply fine tuning the length slightly should take care of indexing.

Yeah, I guess so. The 77 doesn't have an especially long action as standard and can be difficult to mount scopes on so shortening will further limit the options but by no means be impossible.

[RustyBuzz]

Thanks for all the useful input guys.

I plan to make a new 32mm cylinder, and attach the end block using a plug. I feel that this is a much better solution for a rifle that will be in a constant state of experimental flux.

Ideally I will make the action as short as possible, to which Shed’s solution makes a lot of sense. If I can shorten the action by as much as 20mm I would be very happy.

I guess I was wondering if anyone had worked out a calculation to create a decent starting point?

[hmangphilly]

Soz chaps, let me clarify.

My intention is to make a smaller, lighter, better handling 77. Ideally around the same size as a Sport. I have no experience with anything under 25mm so was wondering roughly how much room I need to allow for the spring.

Richard

If the primary intention is to shorten the action , why not stay with the 25 /26 bore and shorten the stroke .?
You can play with set ups in the gun now to see if you like the feel

How much do you think you want to take off the length ?

[RustyBuzz]

If the primary intention is to shorten the action , why not stay with the 25 /26 bore and shorten the stroke .?
You can play with set ups in the gun now to see if you like the feel

How much do you think you want to take off the length ?

The main aim is a much smaller, faster handling rifle. Most of this will be achieved with the reduction to a 32mm external diameter, hence the 24mm bore in the compression chamber.

As I say, a 20mm reduction in length would be fab as this will make a massive difference to the rifles CG.

[Shed tuner]

The main aim is a much smaller, faster handling rifle. Most of this will be achieved with the reduction to a 32mm external diameter, hence the 24mm bore in the compression chamber.

As I say, a 20mm reduction in length would be fab as this will make a massive difference to the rifles CG.

like an HW57 with a proper sliding comp tube... in fact, why not use a 57 action ? Machine out the breech area, make a comp tube for it, that'd work. The 57 is quite nice, except for the variable accuracy due to the loading gate - some are great, some are crap.