Rifle empty weight and march energy cost over 3 days with a loaded pack

1Introduction

This study asked whether rifle mass added to the energy cost of a three day approach march with a loaded pack. The question mattered because rifle mass was one component of carried load, and the paper measured its contribution within a defined march scenario rather than fatigue itself.

The analysis used GunSpec catalogue records for the HK416, AK-74 and FN SCAR-L, together with the documented load carriage equation and the ammunition per kilogram analysis. Figure 1 shows the empty weight ranking used in the study, with the FN SCAR-L at 3,500 g¹, the AK-74 at 3,300 g² and the HK416 at 3,020 g³. Table 1 and Equation 1 then converted those carried masses into march energy cost for 20 km⁴ and 30 km over 3 days, with kit held at 25 kg, terrain set to a dirt road and grade at 0%. Table 2 and Equation 2 assessed ammunition carried per kilogram for a 5 kg⁵ budget.

The paper therefore measured two linked quantities, march energy cost and ammunition per kilogram, using catalogue figures and documented calculations. It did not measure fatigue, performance or injury.

2Data

The firearm records gave empty weights of 3,020 g³ for the HK416, 3,300 g² for the AK-74 and 3,500 g¹ for the FN SCAR-L. Figure 1 shows the same ordering, with the FN SCAR-L heaviest and the HK416 lightest.

• (a) HK416
• (b) AK-74
• (c) FN SCAR-L

The march scenarios used 20 km⁴ or 30 km, kit at 25 kg, dirt road terrain and 0% grade. The carried mass in the march analysis was 5.82 kg for the HK416, 5.4 kg for the AK-74 and 6.93 kg for the FN SCAR-L at 20 km, and the same carried masses at 30 km.

The ammunition analysis used a 5 kg⁵ budget. It reported 75 rounds per kg for the HK416, 100 rounds per kg for the AK-74 and 61.2 rounds per kg for the FN SCAR-L. The basic load was 210 rounds for each rifle, with estimated masses of 2,532 g for the HK416 and FN SCAR-L, and 2,362 g for the AK-74. The AK-74 record carried a data warning, so that estimate requires caution.

The data in this chart

Firearm	Empty weight (g)
FN SCAR-L	3,500
AK-74	3,300
HK416	3,020

3Method

The load carriage analysis used Equation 1, the LCDA equation for the metabolic rate of walking under load, with the study grid fixed at 20 km⁴ or 30 km, 25 kg of kit, dirt road terrain and 0% grade. Carried mass was taken from each firearm record, namely 5.82 kg for the HK416, 5.4 kg for the AK-74 and 6.93 kg for the FN SCAR-L. The one day energy cost from Equation 1 was then multiplied by 3 for the three day total, as stated in the study design. Equation 1a gave the worked example for the HK416 over 20 km on dirt road.

The ammunition analysis used Equation 2, rounds carried per kilogram of loaded magazines, with a 5 kg⁵ budget and the catalogue magazine and cartridge figures. The loaded magazine masses were 400 g for the HK416, 300 g for the AK-74 and 490 g for the FN SCAR-L, and the corresponding rounds per kilogram were 75, 100 and 61.2. Equation 2a gave the worked example for the HK416. The AK-74 record was retained with its stated data warning, so its ammunition mass figures were treated with caution.

The method did not model recovery, sleep, climate or change of pace across the 3 days. It also treated the firearm mass as carried on the body within the march model, and did not model a firearm carried in the hands apart from the same mass carried on the body.

The study made 3 requests to the public analysis endpoints (load carriage (the energy cost of a march), ammunition per kilogram), each comparing every firearm of the study in one scenario. The requests listed with this paper reproduce its figures; every other scenario is the same request with its figures changed.

$$M=M_{\mathrm{rest}}+W[0.19+\eta (1.78S^{0.58}+0.27S^4+M_{\mathrm{grade}})]$$

$$\times [1+1.96{\left(\frac{L}{W}\right)}^{1.36}]$$(1)

A worked example, for the HK416, 20 km on dirt road in the first scenario of the study:

$$M=96.7+80.0$$

$$\times [0.19+1.1(1.78\times {1.333}^{0.58}+0.27\times {1.333}^4+0.00)]$$

$$\times [1+1.96{\left(\frac{30.82}{80.0}\right)}^{1.36}]$$(1a)

$M=519.5\mathrm{W}$$t=\mathrm{15,000}\mathrm{s}$$E=\mathrm{1,862.6}\mathrm{kcal}$

$$r=\frac{c}{m_{\mathrm{loaded}}}$$(2)

A worked example, for the HK416 in the first scenario of the study:

$$r=\frac{30}{0.400}$$(2a)

$r=75.0\mathrm{rounds/kg}$

4Results: march energy cost

Table 1 shows the march energy cost for the three rifles under the same march conditions, with 20 km⁴, 25 kg of kit, dirt road terrain and 0% grade. At 20 km, the AK-74 had the lowest daily energy cost at 1,852.8 kcal, the HK416 was 9.8 kcal higher at 1,862.6 kcal, and the FN SCAR-L was 35.8 kcal higher at 1,888.6 kcal. Over 3 days, the totals were 5,558.4 kcal, 5,587.8 kcal and 5,665.8 kcal respectively, which are the one day figures multiplied by 3.

The same ordering appears at 30 km. The AK-74 again had the lowest daily energy cost at 2,779.2 kcal, the HK416 was 14.7 kcal higher at 2,793.9 kcal, and the FN SCAR-L was 53.8 kcal higher at 2,833 kcal. Over 3 days, the totals were 8,337.6 kcal, 8,381.7 kcal and 8,499 kcal respectively, again as one day figures multiplied by 3.

The figures show a small increase in metabolic energy cost with greater carried rifle mass in this scenario. The difference between the lightest and heaviest rifle was 0.48 kg⁶, and the corresponding daily energy cost difference was 35.8 kcal at 20 km and 53.8 kcal at 30 km. These figures are energy costs only, not fatigue.

Firearm	Distance (km)	Carried (kg)	Energy per day (kcal)	Over lightest per day (kcal)	Over 3 days (kcal)	Over lightest over 3 days (kcal)
HK416	20	5.82	1,862.6	9.8	5,587.8	29.4
AK-74	20	5.4	1,852.8	lightest	5,558.4	lightest
FN SCAR-L	20	6.93	1,888.6	35.8	5,665.8	107.4
HK416	30	5.82	2,793.9	14.7	8,381.7	44.1
AK-74	30	5.4	2,779.2	lightest	8,337.6	lightest
FN SCAR-L	30	6.93	2,833	53.8	8,499	161.4

5Results: march energy cost at 30 km

At 30 km⁴ on a dirt road with 25 kg of kit and 0% grade, Table 1 showed the AK-74 with the lowest march energy cost, at 2,779.2 kcal per day. The HK416 was 14.7 kcal higher, at 2,793.9 kcal per day, and the FN SCAR-L was 53.8 kcal higher, at 2,833 kcal per day.

Over 3 days, the same ordering remained because the study multiplied the one day figure by 3. The AK-74 total was 8,337.6 kcal, the HK416 total was 8,381.7 kcal, and the FN SCAR-L total was 8,499 kcal. The difference between the HK416 and AK-74 was 44.1 kcal over 3 days, and the difference between the FN SCAR-L and AK-74 was 161.4 kcal over 3 days.

These figures describe metabolic energy cost only. They do not measure fatigue, and the 3 day totals did not include recovery, sleep, climate or pace change. Figure 1 shows the empty weight ranking used in the study, with the FN SCAR-L heaviest, the AK-74 next and the HK416 lightest.

6Results: ammunition per kilogram

The ammunition per kilogram analysis in Table 2 gave 75 rounds⁵ per kg for the HK416, 100 rounds per kg for the AK-74 and 61.2 rounds per kg for the FN SCAR-L. With a 5 kg budget, the catalogue figures gave 360 rounds for the HK416, 480 rounds for the AK-74 and 300 rounds for the FN SCAR-L. For the basic load of 210 rounds, the estimated masses were 2,532 g for the HK416 and FN SCAR-L, and 2,362 g for the AK-74.

The AK-74 record carried the stated data warning, so its ammunition mass figure required caution. The estimated masses were derived from the catalogue cartridge and magazine figures, and were not recorded weights. Figure 2 shows the cartridge lengths used in the ammunition analysis, 57 mm⁷ for 5.45x39mm and 57.4 mm for 5.56x45mm NATO. The figure shows a 0.4 mm⁶ difference between those cartridge overall lengths.

Firearm	Loaded magazine (g)	Rounds per kg	Rounds in budget	Basic load, estimated (g)
HK416	400	75	360	2,532 (2,316 to 2,747)
AK-74	300	100	480	2,362 (2,161 to 2,564)
FN SCAR-L	490	61.2	300	2,532 (2,316 to 2,747)

The data in this chart

Cartridge	Overall length (mm)
5.45x39mm	57
5.56x45mm NATO	57.4

7Limitations

The march figures in Table 1 were metabolic energy costs from Equation 1, not fatigue. The study therefore did not measure fatigue, performance or injury, and it did not support any claim about those outcomes. The three day totals were one day figures multiplied by 3, so the results did not model recovery, sleep, climate or any change of pace across the 3 days.

The march model also treated carried mass as body borne load. A firearm carried in the hands was not modelled apart from the same mass carried on the body. Figure 1 and Table 1 therefore describe the effect of recorded mass within the stated carriage assumptions, and no broader inference follows from them.

The ammunition figures in Table 2 and Figure 2 were derived from catalogue records. The AK-74 record carried a data warning, because the derived full magazine was lighter than its rounds alone were estimated to weigh, even within the model error. The ammunition mass figures for the AK-74 therefore require caution, and any comparison that depends on that record is limited by the catalogue inconsistency.

• The march figures are the metabolic energy cost given by LCDA backpacking equation with the LCDA graded-walking term for the carried mass. Energy cost is not fatigue: this paper states no fatigue, performance or injury figure of its own, and reports one only where it is quoted from a published source.
• A firearm carried in the hands is not modelled apart from the same mass carried on the body.
• The figures over 3 days are the one-day figures multiplied by 3. No recovery, sleep, climate or change of pace between days is modelled.
• Figures labelled estimated come from a fitted cartridge-mass model and are given as a range; they are not recorded weights.
• Catalogue figures are as the records state them on the day they were read; derived figures are arithmetic on those records, and a record's error carries into every figure derived from it.
• Data warning, AK-74: The derived full magazine is lighter than its rounds alone are estimated to weigh, even allowing the model's whole error: one of the record's two weights is wrong, so the figures derived from it are too.

8Conclusion

The figures show that rifle mass added a small amount to march energy cost in this scenario. In Table 1, the AK-74 had the lowest one day energy cost at 1,852.8 kcal⁴ over 20 km and 2,779.2 kcal over 30 km, the HK416 was higher by 9.8 kcal and 14.7 kcal, and the FN SCAR-L was higher by 35.8 kcal and 53.8 kcal. Over 3 days, the same ordering remained, with totals of 5,558.4 kcal, 5,587.8 kcal and 5,665.8 kcal over 20 km, and 8,337.6 kcal, 8,381.7 kcal and 8,499 kcal over 30 km. Figure 1 shows the same empty weight ordering, with the FN SCAR-L heaviest, the AK-74 next and the HK416 lightest.

The figures do not support a stronger claim about fatigue. The march model reported metabolic energy cost only, not fatigue, and the 3 day totals were one day figures multiplied by 3 with no recovery, sleep, climate or pace change modelled. Table 2 and Figure 2 concern ammunition mass and rounds per kilogram, but they do not alter the march result. The literature step found no verifiable source for a fatigue figure specific to this question beyond the quoted general statements on load carriage stress.

Author contributions

GunSpec Research team: conceptualization; data curation; formal analysis; methodology; visualization; writing, original draft; writing, review and editing.

Funding information

This work was funded by Buun Group Pty Ltd, which publishes GunSpec.

Conflict of interest statement

The authors work for Buun Group Pty Ltd, which publishes the GunSpec catalogue and sells access to the API the figures were computed with.

Data availability statement

The figures come from the public GunSpec catalogue, as read on the date given in the record above. Every figure can be reproduced with the requests in Appendix A.

References

1. GunSpec (2026). FN SCAR-L. GunSpec catalogue record, read 2026-09-30. https://gunspec.io/en/firearms/fn-scar-l. The record cites modernfirearms.net, militaryfactory.com, weaponsystems.net. API: GET /v1/firearms/fn-scar-l. ↩ ↩²

2. GunSpec (2026). AK-74. GunSpec catalogue record, read 2026-09-30. https://gunspec.io/en/firearms/ak-74. The record cites militaryfactory.com, weaponsystems.net, modernfirearms.net, globalsecurity.org. API: GET /v1/firearms/ak-74. ↩ ↩²

3. GunSpec (2026). HK416. GunSpec catalogue record, read 2026-09-30. https://gunspec.io/en/firearms/hk416. The record cites heckler-koch.com, militaryfactory.com, old.weaponsystems.net, en.wikipedia.org. API: GET /v1/firearms/hk416. ↩ ↩²

4. GunSpec (2026). Methods: Load carriage. GunSpec documentation. https://docs.gunspec.io/en/methods/load-carriage. The metabolic cost of a march is computed from the carried mass (the record's empty weight, a loaded magazine as its loaded weight minus its empty weight, attachments and kit) by the published equation the article names, with the march assumptions it states. It is an energy cost, not a measure of fatigue or performance. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

5. GunSpec (2026). Methods: Ammunition per kilogram. GunSpec documentation. https://docs.gunspec.io/en/methods/ammo-load. A full magazine is the record's loaded weight less its empty weight, and rounds per kilogram and a weight budget are counted from it; a basic load's mass is estimated by the fitted cartridge-mass model and given with that model's error as a range. ↩ ↩² ↩³ ↩⁴

6. GunSpec (2026). Methods: Comparisons. GunSpec documentation. https://docs.gunspec.io/en/methods/compare. A difference is one catalogue figure minus another of the same kind, in the same unit. ↩ ↩²

7. GunSpec (2026). 5.45x39mm. GunSpec catalogue record, read 2026-09-30. https://gunspec.io/en/calibers/5-45x39mm. The record cites en.wikipedia.org. API: GET /v1/calibers/5-45x39mm. ↩

8. GunSpec (2026). 5.56x45mm NATO. GunSpec catalogue record, read 2026-09-30. https://gunspec.io/en/calibers/5-56x45mm-nato. The record cites en.wikipedia.org. API: GET /v1/calibers/5-56x45mm-nato.

9. GunSpec (2026). Methods: Sources. GunSpec documentation. https://docs.gunspec.io/en/methods/sources. Every figure in this article was traced by a program to the records above, allowing only rounding and unit conversion, before a person approved it for publication.

10. Looney DP, Lavoie EM, Vangala SV, Holden LD, Figueiredo PS, Friedl KE, et al. (2022). Modeling the Metabolic Costs of Heavy Military Backpacking. Medicine & Science in Sports & Exercise 54(4):646-654. https://doi.org/10.1249/MSS.0000000000002833

11. Looney DP, Santee WR, Hansen EO, Bonventre PJ, Chalmers CR, Potter AW (2019). Estimating Energy Expenditure during Level, Uphill, and Downhill Walking. Medicine & Science in Sports & Exercise 51(9):1954-1960. https://doi.org/10.1249/MSS.0000000000002002

12. Soule RG, Goldman RF (1972). Terrain coefficients for energy cost prediction. Journal of Applied Physiology 32(5):706-708. https://doi.org/10.1152/jappl.1972.32.5.706

13. Cunningham JJ (1991). Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation. American Journal of Clinical Nutrition 54(6):963-969. https://doi.org/10.1093/ajcn/54.6.963

14. Boffey et al. (2018). The Physiology and Biomechanics of Load Carriage Performance. Military Medicine. https://doi.org/10.1093/milmed/usy218

15. Graham et al. (2025). Novel approaches to evaluate characteristics that affect military load carriage. BMJ Military Health. https://doi.org/10.1136/military-2024-002899

@techreport{gunspec2026rifle,  title       = {Rifle empty weight and march energy cost over 3 days with a loaded pack},  author      = {{GunSpec Research team}},  institution = {GunSpec.io},  year        = {2026},  month       = oct,  url         = {https://research.gunspec.io/en/research/rifle-empty-weight-and-march-energy-cost-over-3-days-with-a-loaded-pack-4},  note        = {Not peer reviewed}}