Any Interest in 6mm ? - Build Thread

[rsterne]

Here is what the finished reamer looks like....



It was hardened, polished, and them tempered for 30 min. at 465*F, raising the temperature to 480*F for the last 5 minutes because it was a little too pale a straw colour.... The result is a nice brown, the way I like my reamers....

Bob

[subscriber]

Nice, Bob! An that would make it about Rc 60?

[rsterne]

No idea....

Bob

[subscriber]

No idea....

Close:  http://www.buffaloprecision.com/data_sheets/DSO1TSbpp.pdf

[rsterne]

Just found that, because you made me curious.... Here is their tempering chart for O1 tool steel, which is what I used....



So, yep, about RC 60 would seem correct.... not that I have any real idea if that is the best for the job or not.... *LOL*....

Bob

[rsterne]

It's always a nervous time when you first use a new chamber reamer.... They seem to get smaller through the polishing, hardening and repolishing, and I'l always wondering if it will cut the chamber to the right diameter.... When I set up to cut the chamber in my 6mm barrel today and starting advancing the reamer, it went so easy I thought "carp, it's too small".... I cut the leade into the barrel about 1/4", and then pulled it out.... To my surprise, and delight, I found that it cut better than any reamer I had ever made before, and the little channel between the flutes that I had filled with oil had a nice pile of shavings sitting in it....



Usually you can "feel" quite a bit of resistance on the handwheel on the tailstock as you feed the reamer in, the barrel gets warm, and the shavings are not much more than dust size.... Not so with this reamer, the shavings were like little needles, about 1/16" long and 0.005" in diameter or so.... I could actually feel the "bump-bump-bump" as each flute hit the rifling lands and slowly carved them away.... I don't know why this one is so much better.... but I hope they all come out like this from now on.... It may be that I honed it just right.... I know I will try the same honing technique on another reamer the next time, to see if that was the difference.... I sure hope so, this one cut a beautiful chamber in short order.... Here are the three 6mm bullets I used to set the length....



Left to Right are the 63 gr. NOE 245-64 FN.... the 61 gr. Bowman by Arsenal.... and the 73 gr. NOE 245-74 FN.... The 63 gr. is a shortened version of the 73 gr., with the gas check shank missing.... These two bullets are "bore riders", and you can see the shiny spots on the nose of the 73 gr. where it rubbed along the lands.... The front driving band is engraved to about half the rifling depth, as it is sitting in about the middle of the leade.... There are very slight marks on the rear band from the back of the leade, and about the same on the back band of the Bowman (which is a copy of the Lyman 257420, scaled down to 6mm).... The middle band of the Bowman is engraved about half depth, and the very front of the front band is just about full rifling depth, which is what I aim for.... The 63 gr. barely touches the leade with the front band, but if you push slowly on it, you can feel that it is seated firmly against it, but just enough to leave tiny marks.... There are some tiny polished spots on the nose, just at the back of the ogive, where it is just sitting on the lands.... I'm very pleased with the way this chamber came out.... In terms of size, a bullet sized to 0.243" slides in easily.... but one sized to 0.244" requires a bit of a push.... so the chamber size is perfect.... This reamer is a KEEPER !!!

This afternoon I set up the receivers in my milling attachment in the lathe, and adjusted it so that it cut a 20 MOA angle into the top of the Dovetails.... That works out to only about 1mm (0.040") less height at the front of the dovetail than at the rear.... but should put the POI at about optical center of the scope at 100 yards.... I then machined the dovetails into the sides of the raised portions on the receivers.... I took a bunch of photos of the two receivers with the dovetails, and had trouble with reflections hiding the angle of the dovetails.... but you can just about see that in the photo below (with a little imagination)....



If you look closely, you will see that at the front of the dovetail, the bottom of the cutter ran along the top edge of the bevel on the corners of the receiver.... but at the back of the dovetail the cutter left a ridge 1mm above the top of the bevel.... It's hard to believe that is all the elevation angle required of the barrel, relative to the scope, to put you on target at 100 yards.... but 0.040" in 6" works out to 24" at 100 yards (0.040 x 100 x 3 x 12/6).... When you mount the scope you can barely see the angle, if you didn't know it was there you would never notice it....

Bob

[subscriber]

So, yep, about RC 60 would seem correct.... not that I have any real idea if that is the best for the job or not.... *LOL*....

That is over twice the likely hardness of the barrel; so, perfect.  Besides, O1 has been used for a long time.  It is almost on par with high speed steel; except that if you overheat O1 by cutting too fast (or fast and dry), the cutting edge hardness will be lost.  The way you use your reamers, this is of no concern.

O1's beauty is that it is not expensive, and you can make your own tools.   Now, I have played around with A2 tool steel because you don't have to quench it in oil or water to harden it, so it distorts less from thermal shock.

[subscriber]

double post...

[rsterne]

Barrel is 4130 CrMoly, probably started as normalized tubing, but who knows what the hardness is after hammer forging....

Bob

[subscriber]

Barrel is 4130 CrMoly, probably started as normalized tubing, but who knows what the hardness is after hammer forging....

Bob

Probably no more than 30 Rc at the surface.  Less in the bulk, but more than before hammer forging.

[rsterne]

Over the last 3 days I machined the O-ring groove in the breech of both barrels, and cut the CF sleeve to length and glued it onto the barrel liner with Loctite 638 (green).... After curing 24 hours, I gave it a post-cure in my wife's oven at 175*F for 3 hours.... This greatly enhances the strength of the bond.... Today I laid out and machined the barrel ports, and then installed each barrel in its receiver with a couple of setscrews (just tightened a bit against the CF sleeve), making sure the flat on the barrel port was perfectly centered in the hole in the receiver for the transfer port.... I made a drill jig from a 20mm long x M5 SHCS by drilling a 1/8" hole through it in the lathe.... and then inserted it in each of the holes in turn and drilled through the carbon fibre until I touched the steel of the barrel with the 1/8" drill bit.... I then mounted the barrel in my milling attachment in the lathe, centering off each 1/8" pilot hole, and enlarged the hole to clear the M5 setscrews with a #8 drill.... I drilled deep enough to leave a shallow pocket in the steel liner for each screw.... Here is the way that works....



The piece of tape on the drill bit was just a visual indicator to let me know when I would hit the steel.... The drilled out M5 SHCS worked perfectly as a drill guide, it is a great way to mark setscrew pockets accurately in a barrel.... I deburred the barrel ports and faired the 0.260" round transfer port seat ID into the oblong barrel port which had been machined with a 3/16" end mill.... The port in the 6 mm ended up at 0.194" wide by 0.294" long and the one in the .257 is 0.205" wide by 0.305" long.... which gives me full bore-area porting in both barrels.... Here is a photo of one of the finished barrels installed in the receiver....



The next job is to complete the bolts.... I already have them turned down to diameter, but have yet to install the bolt handles and cut
them to length at both ends.... The barrels also need lapping inside with JB's Bore Paste and polishing with Bore Brite.... I'm getting pretty close to being able to take the first shots....

Bob

[subscriber]

Nifty!

[YEMX]

I can't wait to get started on mine!!! 

All your work looks amazing Bob!

[lloyd-ss]

Bob, I like that drill busing idea for a tapped hole. Very clever!
The build is looking great!
Lloyd

[rsterne]

Final assembly and first shots today.... Yesterday I cut the bolts to length and drilled and tapped them for the bolt handles.... I checked the chambering of all the bullets, and the .257 was OK but the longest 6mm bullet (73 gr.) was too hard to chamber, so I had to shorten the front of the bolt about 0.050" until I could just chamber it.... I then sanded the receivers and cleaned them thoroughly, inside and out.... Today I lapped the bores with JBs Bore Paste and then polished them with Bore Brite.... I made the bolt handles, and cleaned and assembled the uppers for the first time.... Everything worked smoothly, so I measured the required length for the transfer port on  the unregulated gun (I'm still waiting for the regulator to finish the other lower).... It turns out the both the 6mm and .257 will require the same length transfer ports, which means I won't have to make multiples, just one for each lower.... The transfer ports are made from a piece of 3/8" OD Teflon rod, drilled out with a size "F" drill (0.257") and faced flat on the ends to the correct length to have about 0.020" of crush for sealing.... That was the last part required, so I was able to bolt the upper to the lower for the first time.... I put the .257 cal upper on the unregulated lower, and here is what it looks like.... The reservoir is 300 cc, and the gun as shown weighs 6 lb. 9 oz.... Using an aluminum reservoir sure makes a HUGE difference to the weight....



Well, the first shot is always an exciting and nervous time.... My Great White was at 3400 psi, so that was the fill pressure I used.... I sized some 82.5 gr. bullets, about in the middle of the weight range I hope to use, set up my Chrony and fired the first shot.... VERY loud, and 978 fps (175 FPE).... I increased the SSG gap from the 1 turn I had it set to, a turn at a time, and it made NO difference to the velocity, so I was firmly up on the plateau, and using a lot of air, the pressure in the Great White dropping a bit with each shot.... Suddenly I got a huge BRRAAAPPPPP which dropped the pressure about 100 psi.... I backed the SSG out a couple more turns, and it did the same thing.... after a couple more big burps, it was time to try something different.... I removed the SSG and set the preload on the 13 lb/in. spring to zero, and reinstalled it with 1 turn of gap.... by this time the pressure was down to just over 3000 psi, and I got a nice sharp report, still pretty loud, and 933 fps (160 FPE).... I started increasing the gap on the SSG, and the velocity stayed the same, but the gun was getting progressively quieter, but still using a lot of air.... Suddenly on one shot, all I got was a click.... Hammer hit the valve, but the gun did not fire.... I started cranking the SSG back in to increase the hammer strike, but even with zero gap, no joy.... I figured something inside the valve had failed, so I degassed the gun and pulled it all apart.... This is what I found....



The slotted wheel on the front end of the valve had sheared, and the sleeve over the poppet was no longer attached to the valve, but travelling with the poppet.... This meant there was no balancing force left on the poppet, so no wonder it would not open with the very light hammer strike.... If you ever need proof that the ART/SS valve reduces the required hammer strike, there it is.... I have tried to wrap my brain around where the force came from that sheared that wheel, and for the life of me, I can't see how there is any force on it, with the poppet free to slide inside it.... but there obviously is.... I am using a stepped poppet, with a 1/4" small end and a 3/8" big end, inside that sleeve, so the difference in those areas creates a balancing force of 184 lbs. at 3000 psi (258 lbs. at my 4200 psi MSWP).... If that force is "lifting" the poppet away from the seat to reduce the 464 lb. closing force (at 4200 psi) to "only" 206 lbs.... then I guess it must be "pulling" that sleeve downwards (towards the seat) with that same 258 lbs. of force (it has to go somewhere).... That was enough to shear the four very thin webs between the slots in the wheel that was holding it....

The first shots met or exceeded my expectations, but time for a redesign of that wheel.... 

Bob

[Rob M]

the numbers are simply INCREDIBLE!  The gun looks awesome ,and quite utilitarian .. Question , is the tube 2024 or 7075 ?? I know its somewhere in the thread, I just don't recall. As for the valve, I read how you described the failure, I still cant see what started the chain of events. Sounds like a failed oring in the " balanced side " can cause a massive increase in load , enough to snap the web..

[MJP]

Nice Bob!
Did I mention something about the end support strength to you or to Travis?
When making more power you need to consider the internal forces acting against the end on shot cycle. None the less awesome work on the guns.

Marko

Edit: Take the forces at play, and add the full hammer blow to that, followed by the BRRRAPPP and I dont wonder why the end failed. I'm betting the valve bottomed out pretty good on the first shot.

[rsterne]

The tube is 2024-T3 on both guns, but the one on the unregulated gun is 0.120" wall, so the MSWP is 4200 psi at a 3.5:1 safety margin.... The O-rings in the valve were 100% intact, there was NO leakage out the barrel either before or after the valve failed.... and they appear perfect on disassembly.... in fact the sleeve was still just sitting on the poppet like nothing had happened....

Marko, you did indeed warn me against using a snap-ring to retain that wheel, but if I understand what happened correctly, the force on the sleeve is inwards, towards the valve seat.... since the effect of the stepped poppet is to reduce the force in that direction.... equal and opposite, right?.... The wheel sits against a shoulder, the snap-ring simply retains it there.... The four thin vanes between the slots simply sheared off....

The poppet cannot be driven to the limit of it's travel by the hammer, because the hammer hits the end of the valve before the poppet runs out of travel.... In addition, poppets seldom open all the way anyways, and would not travel as far on a "bounce" as the initial strike.... I really doubt if the hammer bounce harmed the valve in any way, or was responsible for breaking the wheel.... It was many, many shots after I eliminated the bounce, and with a much lighter hammer strike, when the wheel broke.... I suppose it could have cracked during one of the BRRAAPPPPs.... but I know for sure the hammer cannot bottom out the poppet, I was very careful about that when I made the valve.... I think the 184 lbs. of load was just too much for the small amount of metal I left in between the slots....

Bob

[oldpro]

 Man does that look failure. I dont have pics anymore but in the ART gate I had a SS 357 Bulldog valve that did almost the same thing so I had to beef it up also. If your getting the Brrraaapp the ratio is still under balanced and can use a closer ratio between 70-80% of diameter of the two pistons but this comes at a price of harder cocking of course. In the Bulldog SS valve it didnt shear it bent or mushroomed till the Thimble rested on the poppet making it impossible to open. The fun never stops with R&D but you have to admit theres always something to learn in our failures. The rifle looks really really awesome by the way.

[oldpro]

 Found the pics. Failed thimble retainer and new thicker retainer that held.