To my knowledge, there is nothing written about min/max thickness nor content of the front bag in the DCRA rules which Canada follows for FClass shooting.
When I shot BR (LV/HV/HBR) there were rules for bags, mix-max and fill content. That said, I never ever saw a bag emptied to see what was in it in BR either. I know most everyone uses either regular sand or heavy sand to fill their bags, both front and rear. You do not want the front bag too hard or you will get hop.
DCRA Rules regarding rests
11.15 Rifle Rests
(1) TR, TR-O and BP Competitions
Rifle rests may be used between shots in deliberate fire TR, TR-O and BP competitions, but
in no circumstances shall they provide any measure of support during a shot. If attached to
the rifle, any rest shall be included in the rifle's total weight. See also Rule 12.10.
(2) 'F' Class Competitions
The rifle may be supported by any means which provide no positive mechanical method for
returning the rifle to its precise point of aim for the prior shot. Rule 11.05(5)(a)(i) also
applies. Subject to the foregoing:
(a) No more than two rests may be used to support the rifle. If two rests are
employed, they may not be attached to each other.
(b) The use of any form of chair, stool or table is prohibited; however, disabled
competitors may apply for appropriate dispensation (see Rules 6.05(3) and
12.25).
(c) A front rest may be employed for either the rifle's fore-end or for the shooter's
forward hand. If attached, clamped or held onto the rifle in any way (eg, a bipod
or tightly clamped benchrest) a front rest must be included in the rifle's overall
weight (see Rule 11.05(5)(a)(ii)). The front rest or bipod may contain integral
mechanism(s) for adjusting it in any direction.
(d) No portion of the rifle's butt or pistol grip (including any integral or attached
jack) shall rest on the ground or touch any surface not visibly deformable by
trigger pressure. Any rear rest employed shall not be attached, clamped or held
onto the rifle in any way (see Rule 11.05(5)(a)(ii)), nor shall it contain an integral
windage adjustment mechanism.
(e) As an alternative to (c) and/or (d), the rifle may be rested upon a simple
central support such as a rolled coat, towel, blanket or groundsheet, or upon a
cushion, sandbag or beanbag.
(f) Any number and type of object(s) (of any material) may be placed beneath
each rest to compensate for variations in the height, slope or surface of the firing
point, also beneath and around any rear sandbag or beanbag to reduce its
rolling.
(g) The rest(s) or base(s) for each rest may have up to three "feet" each
respectively. Each such "foot" may terminate in a spike, which may be pressed
into the ground by up to 50mm (about 2") provided this causes no significant
permanent harm to the firing point.
(h) Rests may be adjusted after any shot, in order to change the point of aim
and/or to compensate for the rifle's recoil having caused such things as rest
movement and/or sandbag/beanbag settling.
(j) Subject to its weight being included in that of the rifle, a sling may be used in
addition to any of the support systems mentioned above.
US Rules..
3.4.1 Rifle Rests -
(a) F-Class Open Rifle (F-O) - The rifle may be supported by any means which provide no positive mechanical
method for returning it to its precise point of aim for the prior shot. Subject to:
(1) No more than two rests may be used. If two rests are employed, they may not be attached to each
other.
NRA Intl Fullbore Prone
(2) The use of any form of a table is prohibited. Separate flat boards or plates not exceeding the dimensions
of the individual rests by two inches are allowed to be placed under the front and/or rear rests. See Rule
3.4.1(a)(1).
No leveling screws or protrusions are allowed on these boards or plates. They must be flat on the top
and bottom.
This discipline is a modification of high power prone shooting, not a form of bench rest and should not
be construed as such
Disabled competitors may apply to the NRA Protest Committee for appropriate dispensation.
The intent of this rule is to prevent the use of a table type device.
(3) A front rest may be employed for either the rifle’s fore-end of for the forward hand. If attached, clamped,
or held to the rifle, the front rest must be included in the rifle’s overall weight (Rule 3.4(a)).
(4) No portion of the rifle’s butt or pistol grip shall rest directly on the ground or on any hard surface.
Furthermore, any rear rest employed shall not be attached, clamped, or held onto the rifle in any
manner. Mechanically adjustable rear rests are not allowed.
(5) As an alternative to (3) or (4), the rifle may be rested on a simple central support such as a rolled jacket,
towel, blanket, or groundsheet, or upon a sandbag or beanbag.
(6) Any number and type of objects may be placed beneath each rest to compensate for variations in the
height of slope of the firing point or to reduce its rolling.
(7) The front rest or base may have up to three spiked feet which may be pressed into the ground by no
more than 50mm (approximately 2 inches) provided this causes no significant harm to the firing point.
(8) Rests may be adjusted after any shot to compensate for rest movement or settling. A sling may be used
in conjunction with the rest(s), but its weight will be included in the rifle’s overall weight (Rule 3.4(a)).
(b) F-Class Target Rifle (F-T/R) Rests - A bipod and/or sling are the only allowed front supports for the F-T/R
rifle. The rifle may be supported by a bipod and/or sling and a rear support which provide no positive
mechanical method for returning it to its precise point of aim for the prior shot. Subject to:
(1) The bipod and/or sling and rear support may not be attached to each other.
(2) The use of any form of a table is prohibited. Separate flat boards or plates not exceeding the dimensions
of the individual rests by two inches are allowed to be placed under the front and/or rear rests. In the
case of a bipod, the board may not exceed the width of 24” nor depth of 12” and thickness of 1/2”.
No leveling screws or protrusions are allowed on these boards or plates. They must be flat on the top
and bottom.
This discipline is a modification of high power prone shooting, not a form of bench rest and should not
be construed as such
Disabled competitors may apply to the NRA Protest Committee for appropriate dispensation.
The intent of this rule is to prevent the use of a table type device.
(3) A bipod is a device with no more than two legs that touch the firing point. It must be rigidly attached to
the forend of the rifle. The bipod may have rigid or folding legs, and may be adjustable to compensate
for the uneven surface of the firing point.
NRA Intl Fullbore Prone
(4) No portion of the rifle’s butt or forend shall rest directly on the ground or any hard surface. A rear rabbit
eared bag, small sandbag or a gloved hand may be used to support the rifle’s butt. Any rear support
employed shall not be attached, clamped or held to the rifle in any manner. The rear support may not be
fixed to or protrude into the firing point. Mechanically adjustable rear support is not allowed.
(5) Any number or type of objects may be placed beneath the bipod or rear support, to compensate for
variations in height or slope of the firing point.
(6) The bipod and rear rest may be adjusted after any shot to compensate for rest movement or settling. A
sling may be used in conjunction with the rest(s), but its weight will be included in the rifle’s overall
weight (Rule 3.4.(b)).