SmallCaliberCentral

I would appreciate any insight the collective may have.
First a little about me and my plight—sorry if this gets a little bit long. I am a moderately experienced handloader. Managed to get several rifles in the 1/4 to 1/3 MOA range. Some I have been unable to get into the 1 MOA range. I have spent a bit of time with 1000 yard paper punchers. I live in the good old NJ were shooting whistle pigs must be accomplished with ever decreasing grain weight s of bullets. Currently limited to 55 grain (down from 70 grain a year ago) and .25 caliber max. I also do some shooting elsewhere. I have land where 300 yards is the norm, 600 can happen regularly and I may get out past that. I decided to get my self my first scratch built custom rifle. I am building a bolt action .204 Ruger. My goal is to make it shoot the 55 grain Bergers http://buybergerbullets.3dcartstores.co ... _p_15.html which will be pushing the little .204 a bit, but It should be able to just about keep pace ballistically with an 80 grain Bergers in .22BR and outperform the 77 grain Berger 22BR . (This was my preferred round which is now illegal in NJ-which is a custom, not built for me, used up, light bench gun hand me down).

Barrel twist is handled with a 1:7

My main concern was that the chamber be long enough to handle the extremely long 55 grain bullets which none seemed to be able to answer me satisfactorily. I had specified the Pacific Tool and Gauge .204 Match http://pacifictoolandgauge.com/20-cal-2 ... eamer.htmlchamber reamer that was encouraged here, but the gunsmith felt that the .204 Ruger SAMMI http://pacifictoolandgauge.com/20-cal-2 ... eamer.html chamber will give me better mileage but admittedly has little experience with .204 being a long range benchrest centric smith. The only difference between the two is that the SAMMI has the lead/throat which is 0.0507/0.0512” longer than the Match chamber.

Previous treads have talked about how the lead is too long in the SAMMI spec chamber which causes excessive jump which may affect accuracy. So doing the math which was harder than I thought it would be using the SAMMI spec cartridge, and the Berger 55 grain the Base to Ogive should be between 1.964” (max seating depth ogive at neck) to 2.097” (min seating depth-bullet seated ½ caliber). The SAMMI spec has a base to lead of 1.9757”, and the match reamer is 1.925”. Looking at it I should be able to easily seat so I can touch the lands in the SAMMI chamber or less should I desire. In the Match chamber I will be in the lands no matter what I do. If I look at lighter bullets in the berger range they all have longer bearing surfaces than the 55 grain therefore can be seated even further out.

What am I missing—why would I want the shorter Match chamber at all if no matter the bullet as a hand loader I can get to the lands with room to spare using the SAMMI cahmber?

Thanks in advance

the sammi spec reamers typically have a very long throat which would be good for use with the 55 grain berger bullets. There are many different variations of 204 ruger reamers on the market but i would think you would want at least .070 freebore at a minimum. If you don't mind me asking why are you set on using the 204 ruger if you are primarily shooting the 55 grain bullets? There are a number of larger capacity cartridges that would be much better suited to the job. Also a 7 twist barrel is way overkill, an 8.5 to 9 twist is plenty sufficient for the job.

The Berger site recommends a 1:7. After talking to one of their ballisticians it would more than likely work with an 1:8 at least marginally and probably be a bit touch and go with the 1:9 according to Berger. Additionally there should be some issues with the relatively low velocity that i will probably achieve with the .204 case will affect gyro stability. Funnily enough the stability will only increase downrange as the gyroscopic center moves further forward on this pencil like bullet. The MV will likley be slightly affected by the tighter twist. In some work done by Brian Litz the claim was 0.3% slower velocity between 1:12 and 1:8 in a with a 175 grain .308. If I loose three times that i.e. 1% I should be OK trading that for the additional stability out of the barrel.

I chose a .204 becasue it will require the least work to prepare brass, and will allow me to buy brass or loaded ammo if I end up in a bind. If I can't get it to work well enough I can always re chamber for one of the bigger more PIA cartridges. Working up my potential load data I should be pretty close to the performance of an 80 grain 22BR which is no slouch.

Back to the chamber question i calculated the seating depths for all three Berger .20 bullets, and the long 55 grain has a substantially shorter seating depth potential due to its very short bearing service which was totally counter-intuitive to me. My difficulty is that without drawing everything in CAD is that I am afraid that my napkin math may be way off. The 35 grain flat base Bergers have a very long bearing surface compared to the 55 grain which allows it to be set way out in the neck compared with the 55 grain.

sounds like you have it all figured out which begs the question of why ask in the first place.

My apologies if I came across as one of those people. Am I one of those people?

My concern with the chamber question is what I think I have figured out flies in the face of my intuition and some of the guidance that has been proffered here at least for lighter weight bullets and I can’t seem to work it out another way. I am afraid that I may not be able to see the forest through the trees. I feel that I am pretty sharp but know that there are lots of people who are sharper than I especially with this stuff.

My apologies again. Thanks again

Apology accepted. In my opinion i wouldn't over think things on the reamer design. I have tested out many 20 caliber cartridges and even designed a few wildcats of my own. If you really want to go with a custom 204 Ruger reamer designed for the 55 berger bullets i would recommend contacting the good folks at JGS Precision and speak with them about some design ideas.
I would also mention one more time that you may consider a larger capacity cartridge for use with the 55 Berger bullets as i think you will be much happier in the long run. The benefit of going to the 55 Berger is higher BC, the downfall of the 204 ruger and this particular bullet is that you loose a ton of velocity which is one of the main features of the 204 ruger cartridge. When you start giving up velocity to gain BC its really a negative trade off until you get out well past 500 yards. If you really want to stick with the 204 Ruger you might consider running the 39 Sierra Blitzking with a .287 BC, with this bullet you can still maintain a velocity of at least 3,700 -3,800 fps and a very respectable BC for the given bullet weight.
I am currently working on a PPC based 20 caliber wildcat that I designed for use with the 39 Sierra Blitzking, it will push the 39 sierra to 4,000 fps + with less than 28 grains of powder, this in turn creates a very enjoyable and efficient target/varmint cartridge out to 500 + yards with very minimal holdover.