Sand Bag Tuning for Best Results — Fill Type, Levels, Mixing
This discussion of rear bag designs and fill levels may offer some new insights for many readers. By “tuning” your rear bag you can reduce hop on shot-firing and help your rifle track better. All that can translate to better scores, particularly with large-caliber rifles.
Tuning Your Rear Sand Bags
Over the years, noted gunsmith and a Benchrest Hall-of-Fame inductee Thomas ‘Speedy’ Gonzalez has learned a few things about “tuning” rear sandbags for best performance. On his Facebook page, Speedy recently discussed how sand bag fill levels (hard vs. soft) can affect accuracy. Speedy says you don’t want to have both your front and rear sandbags filled up ultra-hard. One or the other bag needs to have some “give” to provide a shock-absorbing function (and prevent stock jump). And you want to tune your fill arrangements to match your shooting style. Free recoil shooters may need a different fill levels than bag squeezers (who a softer bag but harder ears).
SAND BAGS & HOW TO FILL THEM by Speedy Gonzalez
I was asked several times by competitors at the S.O.A. Matches and F-Class Nationals as to how I fill my sand bags for benchrest competition. Here is a copy of a reply I gave several years ago:
Back in the old days, Pat McMillan told me: “You can not have two bags filled so hard that you gun bounces on them in the process of firing round at your target, especially if you have a rig with a very flexible stock. The bags must be set up in a manner for them to absorb the initial shock of the firing pin moving forward and igniting the primer.
Then [they must] maintain their shape and absorb the second shock wave as well the rearward thrust and torque of the rifle. What happens to the rifle when this is not done? Well let me tell you. The rifles have a very bad tendency to jump and roll in the bags. This causes many of those wild, lost shots that one can’t explain.”
Here’s some Good General Advice for Bag Set-up:
1. You should not have TWO hard bags [i.e. both front AND rear] in your set-up.
2. Heavy sand magnifies these phenomena.
3. If you are a bag squeezer, pack ears hard and leave bag pliable enough to squeeze for the movement required. You may pack front bag as hard as rules permit.
4. Free recoil shooters pack both bags firm, but not so hard as to allow stock jump. Especially if you have a stock with a very flexible forearm.
5. We use play-ground sand, also know as silica sand. I sift mine to get any large impurities out then mix it with 25% to 50% with Harts parakeet gravel to the desired hardness that I am looking for. The bird gravel keeps the sand from packing itself into that solid as a brick state.
Speaking of bricks — another thing that happens when shooters employ that heavy zircon sand is the ears form a low spot under them from recoil and then tend to rock back and forth with the rifle causing many low shots to crop up. Edgewood makes an Edgewood/Speedy rear bag specially reinforced under the ears to eliminate this scenario.
Shown below are the latest SEB Bigfoot Bags. Note that the bags sit perfectly flat — there is no bulge on the bottom even though the bags are “packed to the brim with sand”.
General Thoughts about Bag Construction and Ear Materials
I do not like the solid double-stitched leather bottoms. While this seems like a good idea, I see more shooters have problems because of them. They tend to slide around the bench and or slide with the rifle on recoil. The standard Protektor with Cordura rabbit ears and an Otto ring bag with a Cordura front would be what I would suggest to the new shooter or one of the Edgewood / Speedy rear bags, these mimic the “Donut” and feature a ring of leather around the bottom circumference that keep the bottom from rocking on the bench or ground[.]One last note –If you use the Cordura bags, keep them sprayed with a good silicon spray or “Rain-Ex”. This keeps them from getting sticky. — Speedy
How to Fill Shooting Sandbags Most Effectively
For most shooting applications, “heavy is good” when it comes to sandbags. The more your sandbag weighs, the better it will resist movement or mis-alignment that can throw off a shot. In order to increase the weight of their rear bags, serious shooters have turned to exotic sand formulations that offer greater density (hence higher weight by volume) than plain silica sand (aka “play sand”).
Many serious Benchrest shooters fill their sand bags with “heavy sand”. This practice may seem “overkill” to most shooters, but its benefits are realized quickly when shooting from a bench. Heavy sand is more dense than normal silica “play sand” and therefore makes the sandbag heavier and more stable. How much heavier is the heaviest sand? Check the chart below and you’ll see that Zircon sand weighs essentially TWICE as much as ordinary play sand. More weight equals more inertia opposing bag movement, plus more gravity-induced “stiction” on the bottom of the bag. In other words, using heavy sand helps your rear bag stay planted on the bench during a string of fire. When you shoot a hard-recoiling gun, the difference between a bag filled with play sand vs. Zircon is very noticeable. The Zircon-filled bag stays put. The play sand-filled bag may not.
Dupont Zircon M (pale tan): 165-175 lb/cu.ft; avg. particle size 0.01″.
Zircon — Heavy-Weight Champion
Chromite — Almost as Heavy as Zircon, and 30% Cheaper
Riverbed Sand — Weighs Less but Costs More than Chromite
I would not suggest using the Riverbed sand unless you really need to fill a sandbag immediately and can’t wait to locate a source for either Zircon or Chromite. Keep in mind that 
