.257 cal Monocoque PCP

[dyotat100]

Because the AF guns don't have a bolt the leade in has to be pretty much perfect. To loose and the bullet will fall back in the breech. To tight and you can't get it in with your finger. 

That's why I get under sized reamers. Can always open up but can't put it back. The leade is cut for longest bullet.

If you want to use the .257 reamer your welcome to.

I'm hoping the 338 leade is deep enough. I have a 185 gr mold on its way. Never thought I would be shooting that bip of a bullet. I did a test and it is going to work good as long as the bullet is a shooter. Should be because it's just a upscale of what I shoot now

[rsterne]

Thanks for the offer, Doug.... My plan is to make a .217 cal reamer for my Disco Double today.... If that goes well, I will make my own for the Monocoque.... I'll let you know if I need to borrow one....

Bob

[Erik Green]

Hello Bob;
I am thinking of building a similar rifle to you monocoque in your .308 BBT.  Would like to base it on the TJ's 1-10 twist .308 liner.  Saw you posting on the NOE site re: BBT bullets twist etc. but would appreciate you input on feasibility.   Like you I want to try for MOA at 300yds.  Practical, or would I be ahead to go with .257? Do not want to commission a custom barrel.

[rsterne]

What you are looking for is certainly possible.... You don't state what barrel length or pressure you may use, and these have a direct relationship on the weight of bullet (SD) that you can drive to a given velocity.... I went into quite a bit of detail on that in my thread on the NOE Forum, to try and help shooters pick a bullet weight, and then corresponding twist rate....  http://noebulletmolds.com/smf/index.php/topic,1137.0.html .... For instance, at 3000 psi in a 24" barrel, if you want about 950 fps, you need to stay to an SD of about 0.17, which in .308 cal is about 115 gr.... On the other hand, at 4000 psi in a 30" barrel, you could go to an SD as high as 0.27 and get a similar velocity.... which would allow you to use up to a 180 gr. bullet.... My 113 gr. BBT will work in a 17" twist, while the 176 gr. needs a 10".... There are three weights currently in the NOE Store, the 135 gr. 155 gr. and 176 gr., all available at.... http://noebulletmolds.com/NV/index.php?cPath=374_378

Bob

[Voltar1]

Hi Bob, curious where you are at with this rifle build?
Love to hear more of it eh!!
Walter

[rsterne]

I now have the bullets cast, and sizing dies are here.... I'm just finishing up another project (my BRod builds) and this is next on the list.... I'm tired of waiting for Sean Pero to make my chamber reamers, so I'm going to make my own.... Once I get the chamber cut so that both the 101 gr. and 112 gr. BBTs will chamber properly, it will be time for the final assembly and first tests.... I'm really looking forward to it !!!

Bob

[rsterne]

Well, I finally got back to working on this project.... The reservoir was empty, I had a very slight leak in one of the O-rings (there are only two in the reservoir), so since it was empty I took the opportunity to pull it apart and replace the 70D O-rings with 90D.... It has been sitting at 4000 psi for 24 hours, so I guess I cured the leak.... I measured the land and groove diameter, finished up my reamer drawing, and today I made the chamber reamer.... I'm very pleased with the way it came out.... Each time I do one of these, it gets just a little nicer and more accurate....



I used a three flute design, like the one I did a couple of months ago for my .22 cal Regulated Disco Double.... It is machined from a piece of 3/8" O1 Drill Rod, hardened, and then polished, and tempered in my wife's oven at 465*F for an hour.... While it was baking I did a bit more reading on tempering, and discovered that it is the thickness of the transparent oxide layer on the polished surface that causes the colour.... It starts at yellow, through dark straw, brown, purple, blue, then a lighter blue, as the temperature is increased.... I was aiming for a dark straw to brown colour, which is about 490*F.... Once the oven was up to temperature, after about 10 minutes, it looked just right, but I decided to let it heat soak for an hour, and it darkened to brown with a few purple spots.... I was a bit worried that I had gone too far, but apparently even though the part isn't getting hotter, the oxide layer gets thicker with time.... One recommendation I found was to use 1 hour per inch of thickness of the part, so I really don't need to heat it for as long as I did.... If anything, it might be a little tougher, and not quite as hard, but I think it will cut just fine....

For those of you not familiar with chamber reamers, the part on the left, with no cutting flutes, is the pilot that guides the reamer by running on the lands.... The reamer is then necked down a bit, and has a 1 degree taper for about half the length of the flutes, and then a parallel section to cut the chamber.... I make my chambers about 0.001" over the groove diameter.... It then steps down again at the end of the flutes.... The only part that really does any cutting is the last half of the taper and the first part of the chamber.... You just keep running it deeper into the barrel until the chamber is the length you need for your bullets.... I will be cutting the chamber on my Monocoque to fit both the 100 gr. and 113 gr. BBTs, which means it will also fit the 98 gr. and 109 gr. HP versions.... I have sized a bunch of them (both lengths) to use for checking the chamber depth, most to 0.257", but a few to 0.256" and 0.258" just so I can judge the fit.... The aim is to have the nose of the shorter bullet starting into the leade, but not have the longer one too difficult to chamber.... 113 gr. on the left, 100 gr. on the right....

 

It's really nice to get back to work on this project after a year of waiting for bullet moulds and a chamber reamer (which never arrived).... It won't be long and I will be able to test fire it for the first time....

Bob

[Buldawg76]

I am sure it was very frustrating to be so close to being able to take the first shot yet not having the last two pieces of the puzzle to complete the build. Gald to see you have it back in the works for completion and test firing for the first time.

Mike

[MJP]

Looking good, was a bit worried when I saw the color of the reamer. Did you use hss drill rod, I usually do use hss without hardening, fast and easy to make with carbide tools.
Looking forward for finally seeing some test shots on your rifle.

Marko

[rsterne]

I used "oil hardening" O1 Drill Rod.... Easy to machine before hardening.... Heat bright cherry red and quench in oil (I use Canola oil), then polish back up before tempering in an oven.... If you rub bar soap on the reamer before heating with the torch it prevents that black scale from forming.... The grey oxide coat polishes off quite easily.... Its glass hard before you temper it.... but really brittle.... According to the info I have, the difference in hardness going from dark straw to purple-brown is less than one Rockwell number.... but the colour it is should be a lot tougher....

Bob

[MJP]

Ok, now I understand, cos I usually use hss it's much simple to do one off tools. And my mill is so rigid it does cut it really easy with tungsten carbide cutters.
Do you sharpen the cutting edges with oil stone?

Marko

[rsterne]

I use a very fine diamond file to just hone the flute where the end of the mill cut it, both before hardening and after tempering.... I don't touch the outside of the reamer the rake is created when you plunge the end mill 0.012" past the centerline (I use 4-5% of the caliber).... The outside is polished before hardening by sanding with 400 grit rotating the reamer in reverse to remove any burrs from milling.... The reverse rotating insures you don't round the cutting edge (if you don't push too hard on the sandpaper).... After hardening I polish the outside lengthwise with 600 grit before tempering....



That is the basic cross section of my reamers.... dimensions are in calibers.... The dotted line is the way I make a single flute reamer.... This view is from the tailstock....

Bob

[MJP]

I usually go little bit more past the centerline to get better relief on the back and more rake, but don't know if it really matters doing a cutter for one or two uses.
Everybody makes things little different and endup pretty much the same result I quess. It's like programming cnc, you can tell at the shop who made what program, everyone has their own fingerprint pattern on the programs.

Marko

[rsterne]

The further you plunge the mill past the centerline, the faster the reamer will cut, but generally not as smooth, and also more load on it.... I'm never in a hurry....

Bob

[rsterne]

Yesterday I reamed the chamber, and arrived at a length where the shorter bullet stopped against the leade about 1/16" before being chambered fully, and the long bullet was about 3/32" from being chambered.... This meant you could just feel slight resistance on the bolt handle to chamber the 100 gr., and a slight push to chamber the 113 gr.... That was the last bit of machining I had to do, other than machine the transfer port to final length.... This morning I gave a final lapping to the crown, and then polished the bore with JB Bore Paste, followed by Bore Brite, cleaned the gunk out of the bore, and the gun was ready to assemble and test fire for the first time.... That happened this afternoon, nearly a year later than I had hoped....

My Great White was at 4000 psi, which was about where I hope to run the gun, so I just tethered it and ran the initial tests.... I used the lightest and heaviest bullets I will be using, the 98 gr. HP version of the 3.5 caliber long BBT, and the 113 gr. FN version of the 4.0 caliber long BBT.... With the velocity adjuster bottomed out I couldn't quite cock the gun, but with it backed off 1/2 turn the gun would cock fine, and the cocking effort was still quite reasonable.... This gun has a separate cocking handle on the hammer, all the bolt does is load the bullet into the chamber.... There are so many things different about this build, although I had high hopes, I really didn't know if they would be realized.... It was with those very mixed emotions I pulled the trigger for the first time.... and was rewarded with a velocity of over 1000 fps.... I did the typical testing I always do with a new gun, find out where the maximum is (the velocity plateau) and then work back from there.... I then changed to the heavier bullet, and worked my way back up to maximum.... I only shot 1 or 2 bullets at each 1/2 turn preload adjustment, and then plotted the curves, as shown below....



The maximum velocity is reached  at this pressure with 1.5 turns of adjustment left with the heaviest bullet, and 2 turns left with the lightest one.... The peak performance was 1003 fps (219 FPE) with the 98 gr. bullet, and 950 fps (227 FPE) with the 113 gr.... This is basically right what I had hoped for, what I really didn't know was the pressure it would take to achieve those numbers.... Now I know it takes 4000 psi.... I shot a few more rounds, and did a bit of playing around to see what I could get for a shot string.... I'm not interested in detuning this gun for a long string, the gun will primarily be shot tethered to a regulator, and tuned for a short (3-5 shot) string off reg.... Tuning in this way keeps the efficiency reasonable, and yet enables you to disconnect for a short string if necessary.... Once the tank was down to 3800 psi, and I had a rough idea of where to adjust the preload to get a short string, I shot the following off reg....



That is 4 shots, averaging 928 fps (209 FPE) within a 14 fps ES (1.5%).... The pressure started at 3790 psi and ended at 3320 psi, and the reservoir on the Monocoque is 24.7 CI (405 cc), so that works out to an efficiency of 1.04 FPE/CI.... Let me tell you, it's about twice as loud on the 4th shot as on the first, so when tethered it will be quite a bit better than that four shot untethered average.... Now that I know the gun is a success, I can work on the last piece I need.... an inline regulator that I can adjust from 3000-4000 psi by changing the shims.... That will allow me to fine tune the velocity sweet spot with the pressure, while setting the hammer spring reload to give me that 3-5 shots off tether that I desire....

During this build, I had a lot of negative comments about the 7" twist being too fast, that it would prevent me from ever achieving my goal.... and the rifling being too deep (it's 0.001" deeper than a normal .257 barrel), which would likewise ruin the performance.... I think these initial results should be enough to put the nay-sayers to rest.... I'm just grinning from ear to ear that I achieved my goal, without having to exceed 4000 psi....

Bob

[Gippeto]

Very cool Bob.

Congrats!

Al

[Matt15]

Congrats!! I hope you get to do some long range shooting soon! 

[rsterne]

The plan is to have it ready for Gopher season in the spring.... It is a relief to see I haven't been spinning my wheels for nothing....

Bob

[Buldawg76]

That's some very well deserved performance from the build and I can see that ear to ear grin on your face for sure. I am just amazed at the precision that you put into your builds and reminds me of the hot rod days of my past where every little detail was never overlooked in that search for every last little ounce of go power.

Cant wait to see the accuracy out of it as I am sure it will be the gophers worst nightmare.

Mike

[MJP]

Nice Bob, really happy for you. Will be interesting to see how she groups.

Marko