The formidable weapon permanently aimed at a supposed ally is silent testimony to a relationship that is crucial to Helmand's future but remains a volatile mix of dependence, mutual admiration and deep distrust.
The Afghan National Army (ANA) 215th Corps is now 17,000-strong, gaining in confidence and competence and, according to local polling, generally well respected. It has managed to hold on to the main population areas in Helmand in the face of a Taliban attempt to retake lost territory and as a result the province's towns are much safer than they were a year ago.
But the ANA still cannot fight on its own. Only one of the 215th Corps' four brigades is anywhere near full battle readiness. In fact, a Pentagon report in early December revealed that only one of the ANA's 23 brigades across the country had reached that point. The Taliban's success in infiltrating its ranks has contributed to the number of "green-on-blue", or insider attacks, in which Afghan troops turn their guns on their foreign mentors. There were 12 such attacks in Helmand in 2012, all fatal, sawing away at the bonds of trust on which the Nato exit strategy is based.
Even more importantly, there are early signs that the ANA may be struggling to hold the line on a critical front in the war: the ability to protect Afghan civilians from the Taliban.
The latest UN figures show that the Taliban are now responsible for 84% of civilian deaths in Afghanistan, while the government and its foreign allies are responsible for only 6% (10% are unattributable). And the data for the August-October period shows a dramatic spike in those killings, up 28% from the same period last year, suggesting that ordinary Afghans are becoming more vulnerable as the Afghan army takes responsibility for protecting them.
Brigadier General Ghulam Farouq Parwani, deputy commander of the 215th Corps and a 30-year veteran of Afghanistan's many wars, insists his men can handle the Taliban threat on their own after US and British combat troops leave Helmand by the end of 2014. But only if they are given the tools for the job.
"If the promises made to the ANA are fulfilled, the Taliban will never regroup," Farouq said. "The only thing we lack here is equipment. We need artillery support.
"We have been promised a mobile strike force of 800 men with up-armour [shaped to deflect road mines] and advanced weaponry. And we need our own aviation, because sometimes our coalition partners are busy."
The accelerating departure of the foreign forces is felt everywhere. There were 21,000 US marines in Helmand at the height of the surge last year. Now there are just 6,500. With the departing troops having taken their helicopters and aircraft with them, moving troops around the province is harder. The exodus of US armoured cars is even more noticeable. The 215th Corps suffered 600 casualties in 2012 – 150 dead and 450 wounded, mostly from roadside bombs, or improvised explosive devices (IEDs) – increasingly the Taliban weapon of choice.
Farouq railed against the lack of capacity of Afghan police, locally recruited forces whose development has lagged far behind the army. As a consequence, the 215th has been stuck providing guards for the main roads instead of fighting the Taliban.
"The ANP [Afghan National Police] has limited capability, so our forces are busy with checkpoints," he said. "The police lack organisation, structure and because it is recruited locally, sometimes it has men available, sometimes not. If the ANP becomes capable and does its job, the ANA can get back to fighting."
Farouq's complaints about the police drew the intervention of his US advisers, who suggested he stick to talking about the army. The Afghan general smiled tightly and took up his tirade once more. As the Americans draw down their forces, their clout is visibly weakening.
The mentoring relationship in Helmand has also been hit hard by the insider attacks. It is a small number compared with the total number of daily interactions, but has had a disproportionate impact on trust, as the machine gun demonstrates.
Afghan attempts to impose stricter vetting of recruits has had mixed results. Brigadier Stuart Skeates, the British deputy commander of the International Security Assistance Force (Isaf) in Helmand, believes the problem and its solution may lie deeper.
"Part of the reason why we have had a number of insider attacks, as they're called, is because the soldiers and policemen are susceptible to Taliban messaging; maybe not directly, but they hear stuff on the radio, on the TV," he said. "They hear anecdotes from home. They are susceptible to local mullahs, both at home and here, and are in many cases very suggestible."
Skeates said a British army imam, Asim Hafiz, was leading an effort to counter the Taliban influence among the mullahs but it was a long-term solution, time was short and much hung on the ability of Afghan and foreign troops to trust each other.
The greatest success in Helmand over the past year has been the security of the towns. Even fierce battlegrounds, such as Sangin, are now relatively calm, allowing the return of commerce and the replacement of poppy crops with alternative ones.
Skeates produced a pair of slides showing Sangin town in 2011 and 2012. In the first the circle representing the district centre was largely obscured by coloured dots marking attacks by insurgents. In the second, it was mostly clear.
"We are pushing the violence away from the district centre where the majority of the population live, and that, in a counter-insurgency operation, is incredibly important because if you don't see bombs going off on a regular basis, if you don't hear gunfire near you on a regular basis, your perception is that it is safe. The reality, in fact, is that it is secure," he said.
Across the country, Nato's claim is that 80% of the fighting now takes place in areas where only 20% of the population lives, suggesting that the overwhelming majority of the population is leading a more peaceful life. United Nations data shows that in the first 10 months of 2012 civilian casualties were actually 4% down on the year before – the first time that has happened for a decade. But the rise in the autumn was a source of concern, suggesting the gains are in danger of being eroded.
"The insurgency has also retained its capability to carry out attacks at roughly the same level as last year," the Pentagon report conceded. "Despite the tactical progress of ANSF [Afghan National Security Forces]-Isaf joint operations, the insurgency remains adaptable with a regenerative capacity. It retains the capability to emplace substantial numbers of improvised explosive devices and conduct high-profile attacks."
In Sangin one of the great successes of last year was the election of 33 members of a district community council, the first flowering of local democracy. But within a couple of months four of them had been murdered by the insurgents.
The people went back to the polls to elect replacements but that resilience, and the Afghan army's staying power, will be brutally tested as the Nato troops leave.